Analyzing Starcraft 2's Ranking System
Saturday, July 24, 2010 at 12:29PM 
Let's talk about Starcraft 2's ranking system, specifically the bonus pool system, the focus on ranked matches, and the division system. Before we get into all that, I'll give some background info from three years ago when I talked with Rob Pardo (VP of Game Design at Blizzard) about me possibly taking on the role of ranking-and-tournament systems designer at Blizzard. I ended up having to back out of that race because the Street Fighter HD Remix project suddenly became a reality, and I couldn't pass that up.
The reason I want to tell you about the ranking stuff from back then is to illustrate two points of view--mine and Pardo's--and to explain how it took me three years to understand that his point of view is probably best after all.
TrueSkill vs. Monkeying With Rankings
He asked me to come up with a ranking system for a game, we'll just call it Game X. My first response was that this is easy because it's already a solved problem: Microsoft solved it with what they call TrueSkill. TrueSkill is a refinement of the well-known ELO system used in Chess. One of TrueSkill's main features is that it can apply to games with more than 2 players, while ELO can't. Also, TrueSkill uses a bell curve rather than a single point when referring to a player's skill level. As the system gets more information about the player, it becomes more "certain" that the skill ranking is accurate, and that player's personal bell curve shrinks.
More important than any of those details though, is Microsoft's philosophy about rankings. The premise of their whole system is that players will have the most fun if the ranking system can give matches as close to 50-50 as possible. Yeah it's fun to have to have a few matches that are easy and some that are hard (and you will because of the inevitable variance), but you really do want the ranking system to try to give you close matches. Consider a matchmaking system that simply gave you random opponents, and how from the perspective of a bad player, he just gets stomped repeatedly then probably quits. He is better served by getting evenly matched with other bad players. Over time, he might become a good player rather than quitting.
Microsoft makes another good point here that ONLY winning and losing can be allowed to affect these stats. You can't adjust the matchmaking stat by "experience points" or even by any skill-based stats such as headshots, number of kills, time to finish a lap in racing, etc. All those stats can be gamed, and you will end up trying to get more headshots or something instead of winning. Any formula that equates number of headshots (or any other stat besides wins/losses) with how likely you are to win or lose introduces a layer of imperfect simulation. If we want to know how likely you are to beat someone, we should only consider your wins and losses, and not any in-game stats.
Short version: Microsoft's philosophy is correct (or is it...?), our ranking system should only consider wins and losses and should maximize close matches using TrueSkill, or a close implementation.
But Pardo was not quite on board. Such a ranking system is harsh and while hardcores are ok with it, regular people just get crushed and discouraged. They don't get to see any real progress, he said.
Yes that's true, I responded. And yet competitive games flourish and top the sales charts. We shouldn't ruin our ranking system with some experience point system to help people's egos because that will necessarily make the matchmaking system worse. No matter what anyone claims to want, they will be happier with close matches as those are memorable and intense, and that trumps anything that deviates from the TrueSkill philosophy, I said.
Pardo said that he really wanted what he wanted though, and if he's asking for a system that is more friendly to non-hardcores, then I should be able to come up with one. "If someone put a gun to your head and said to give some sort of incentive to keep people going in a ranking system, what would you do?" Yes, fair enough. If he's asking a question, I should be able to provide an answer. But I still thought the question was more like, "should we ruin the perfect matchmaking a little, a medium amount, or a lot?" while I was thinking, "let's not ruin it at all!"
Later, I presented a system that used a TrueSkill-like number for matchmaking behind the scenes, and that did not deviate by allowing any nonsense stats to affect it. But the UI would prominently display 1 main experience level stat (not the skill-based stat) and 2 other stats that were destined to go up over time as well. (Think "headshots", though it wasn't exactly that. Stats that naturally increase over time.) These secondary stats were dressed up a bit in the form of levels, too, so you basically had three meters going up. The fourth stat, the REAL one, was also displayed (and used for matchmaking!) but it was displayed less prominently. Players had various progress bars to level up, but still perfect matchmaking.
Pardo thought I at least gave it a try, but that I had not tried hard enough. He said "we don't need hyper-accurate matchmaking out of a ranking system. That's not its primary function." This is around the time when Street Fighter HD Remix started, so I did not get the chance to submit a revised system.
What did he mean by that last statement though? Isn't "hyper-accurate matchmaking" exactly the thing we do want from a matchmaking system? Years later I’m willing to say, “maybe not.” It could be a good trade-off to have merely “very good” matchmaking if in exchange for losing “hyper-accuracy” we also gain some fun or keep people playing when they otherwise would have given up. Microsoft’s premise was that hyper-accuracy gives you the most fun possible, but perhaps that premise is not strictly true. Maybe there is a way to add fun without losing much at all in the way of perfect matchmaking. Starcraft 2’s bonus point pool system could be that way.
Starcraft 2’s Bonus Point Pools
The bonus point system is similar to the “rest” system from World of Wacraft. Players accumulate a pool of bonus points over time, capped at some maximum. In World of Warcraft, the function is to give you double the experience points each time you kill a monster, until your bonus points run out. In Starcraft 2 the function is to give you double the *ranking points* each time you get a win, until your bonus points run out. For example, if you were facing a stronger opponent such that winning gives you 10 points and losing would subtract 5 points, then winning would actually give you 20 points. 10 points would drain out of your bonus pool and go into your ranking points (to match the 10 points you’d normally get for winning here). If you lose, you would lose 5 points as usual with no further adjustment.
What is the effect of this system? Some people claim it rewards those who play a lot, but this is looking at it the wrong way. Those people would claim that play-a-lot guy gets more ranking points overall because he uses up all his bonus points all the time, while a play-a-little guy would not play enough to use them up, so he’d get fewer points overall for his wins. True, but missing the point.
The actual play-a-lot guy will play way more than his bonus point pool can hold. Most of his matches will be played without any bonus because the maximum size of the bonus pool must be fairly low for this system to even make sense. Meanwhile play-a-little guy will play most or all of his matches with boosts from these bonus points. So if anything, play-a-little guy benefits. (Though the play-hardly-at-all guy is disadvantaged because he doesn’t use up all those bonus points.) But even THAT isn’t the point.
I think there are two real points to this system. First, it gives you an incentive to play when you otherwise wouldn’t. If you haven’t played in a few days or a week or whatever, you will think “hmm, I could play right now and get the benefit of those bonus points!” And if you wait until the points reach their maximum (in World of Warcraft that’s two weeks, but unknown as of this writing in Starcraft 2), then you’ll have another reason to play: that you would be “wasting” bonus points by not playing. If you just played enough to use up some of the bonus points, you’d be able to keep accumulating more for later. Yes it’s a psychological trick, but at least it gets more people active, which basically helps everyone involved (more opponents).
The second real point hits at something Pardo was originally asking me for: a way to have at least some feeling of progress in the harsh world where actual progress is damn hard. Imagine that Alice, Bob, and Charlie all have 1100 ranking points and that they are all of equal skill, so any matches between them will have a 50-50 chance of going either way. Alice doesn’t play for a while (but her skills don’t decline). Bob and Charlie play each other, and also play even more players of the same ranking/skill. After a while, Alice will still be at 1100 (she didn’t even play). Bob and Charlie will have a HIGHER number of ranking points though, due to the bonus pool. Even if they have even records of 30 wins, 30 losses against each other, they are gaining more points than they are losing because bonus points are added for wins, but there’s no extra penalty for losing. They have the feeling of progress here, even if it’s illusory.
Does this screw up the actual rankings? Well, Bob and Charlie might find themselves at 1300 ranking points (I’m just making these numbers up), so you could say their ranking is inflated. But...not really, because all the other people of their same skill out there have inflated rankings by the same amount. Really, it just stretched the scale out. It’s true that Alice will have bad matchmaking when she decides to play again though. Her rating is 1100, but because of the point inflation that’s gone on, she really “deserves” a higher rating now. That will correct itself pretty quickly though, it just means she’ll have a few matches that are easier than expected when she returns. This is the imperfection in matchmaking that we have to accept if we want the good features of: 1) more people tend to play more, and 2) ego boosts even when you go even against evenly matched opponents.
Also consider that this ego boost is not changing your ranking against a list of specific competitors. If you’re the 1000th best player, you’ll still be that, it’s just that everyone’s points are creeping higher over time. (There’s a reset every season, so it won’t go infinite.) Since your REAL rank as 1000th best player is intact without too much monkeying, it seems ok that the underlying stat increase, as a psychological motivator. We still know who is actually better, so who really cares if everyone’s underlying stat is one that goes up over time or not? Really only your ego cares, so we might as well throw you a bone.
To put it another way, I was thinking about how I have not played Street Fighter HD Remix online in the last two weeks. If I had some bonus pool points waiting for me there, would I have? Probably yes, as dumb as that is. And yet if that made me and thousands of other people play a bit more, it helps the whole scene, so why not.
Focus on Ranked Matches
Microsoft has a clear philosophy on this one too: sometimes people want to be ranked because they are seeking out competition and a way to measure themselves against others, but sometimes people want to avoid all that and play without any fear of shame or losing. Xbox games must have both a ranked mode and an unranked mode (badly named “player matches”). In ranked mode, developers can show any kind of stats to the player they want, while in unranked matches, they are supposed to hide these stats as much as possible. This makes total sense to me, as even I don’t want some record of how bad I am at some game I’m just messing around with. And sometimes I don’t want a win/loss record shoved in my face either.
There’s a tension though, and it goes back to matchmaking. We really want you to play ranked matches a lot so that we can accurately determine your skill level and give you close matches. In Kongai, a card game I designed, we did what some other games do by just renaming the modes. The modes used to be called Ranked and Unranked, but now the ranked match is just called Play Game. This shifts the perception of which one is the one you’re supposed to play. While Ranked sounds ominous and serious, Play Game sounds like it’s for everyone.
Starcraft 2 takes a similar approach here. The default mode is ranked, and it’s not named some scary thing, it’s just presented as the normal way to play the game. The screen where you get into a game devotes only a small space in the corner to creating / joining a Custom Game, which is the equivalent of unranked.
The notion that you’re always playing ranked could turn people off, but it does help matchmaking, like I said. Overall I’m weakly in favor of it.
Divisions
Next, Starcraft 2 does a kind of weird thing though. Within each of the 5 zones, there are a zillion players, so they break up these players into groups of 100. You are giving a ranking relative to those 100 people. So you might be “#25 in the silver league” for example, which is a polite way of saying “#52 billion overall.” I get the point here, it’s another psychological trick to keep you going. Being ranked out of 100 doesn’t sound so bad, and it’s a small enough set that your ranking won’t change wildly for no reason. If you saw your real overall rank (#52 billion), it would change all the time from everyone else moving around you, even if you did nothing.
I think, not sure on this, that Blizzard is also trying to give you a set of people small enough to be in your monkeysphere (more than 150 people is too much for your brain to understand). What’s a bit confusing about this is that based on the beta, you don’t seem to play against these 100 people any more than anyone else. I’m not sure if that was the intent, or an artifact of the beta, or what. Maybe those people happen to not be online when you are looking for a match. For whatever reason, it seemed rare to actually encounter them in the beta, which means you’re really being ranked against 100 random people who you have no idea about. It does sound a bit better if the matchmaking mostly matched you with them, so you got to know them and felt more of a personal connection with your subcommunity, which consists of people at least fairly closely ranked with you. We’ll have to see how this works out in the real game.
One thing I will say about the divisions though, is that the top division SHOULD be designed differently. While average joe is probably very happy with this division system, I think the very top players will not be. Anyone way, way up there wants to know the real, cold, hard truth. They want to know who is #1 and who is #10, etc. They are dying to know this, even. So for the very top bracket only, I suggest that all players are in just one division. I’m not sure if this would be best for the top 1,000 players or 10,000 or 5%, or what the magic number is, but I’m saying people at that end actually want the opposite of what the division system is offering. They really do want to know exactly where they stand relative to each other.
Conclusion
Of all these things, it’s really the bonus point pool system I wanted to mention the most. As soon as I realized that system would have made me play Street Fighter HD Remix in the last couple weeks instead of not playing, I realized that its effect of enticing people to play every so often is more important than any loss of “hyper-accurate matchmaking.” So I think Pardo was right in what he was asking for all those years ago.Sirlin
Followup: Some people have said that the matchmaking stat is not affected by the bonus pool points, and that the bonus pool points are only used for display purposes, like where you appear in your 100 man division. This is not the impression I got from Blizzard's statements, but that claim could be correct. It's also possible the answer to that changed at some point, as who knows what adjustments went on during the beta. If the claim is true, the system matches you by a different criteria than it actually rates you. I don't really know what to think of that.
Sirlin
More information here that the bonus pool points do not affect the "real" underlying stat used for matchmaking, only the ("fake"?) displayed ranking in your division.
Reader Comments (68)
I don't think Blizzard wants to display a Global Ranking to determine the top 100 players overall. Think about it, Blizzard has a monetary stake in running tournament ladders and televised matches. They don't want some anonymous screenname to be sitting on the #1 spot. It would be like if the top 10 online ranked people on HD Remix have names like XxXBALLAZXxX123 none of them show up to any live tournaments. The entire online ranking system gets invalidated because people just assume that the entire list is full of cheaters and boosters.
By keeping the global rankings hidden in SC2, the only way we can determine who are the top players is by watching the tournaments replays; which in turn provides broadcasting revenue back to Blizzard.
@Sirlin: That's an interesting point. We don't _always_ want a "super even" match. Uneven matches can sometimes add "fun value." Lucky for us, this will happen naturally since the system can't always find a dead even match.
The pre-match screen shows "favored," "slightly favored," "even," "opponent slightly favored," and "opponent favored" indicators. Of course, the system won't vary matchups too far apart because of the huge player-base. If a guy outright wants to dominate his opponent (or wants to be dominated), he'd need to join a custom game. Btw, I hadn't thought of how XBox's "rated/player match" naming is dumb, but that's so true!
@Chronocide: In the strictest sense, bonus pool points _do_ cause inflation: meaning points increase but skill-value doesn't. I think where we agree is that this inflated score is still accurate in relation to others--but only if you keep playing!
This brings me to my final point. A hidden matchmaking system is an excellent solution in these huge player-base, automated, unregulated ladders. MMR functions best when it is the most accurate because evenly matched games mean more fun. MMR cannot be the same as what you see on the ladder because accurate skill evaluation isn't exactly conducive to a functioning ladder. Here's why:
1. It makes players who do well afraid to play. They don't want to risk losing their precious points. This is exemplified by chess amateurs' unwillingness to play out their games to a win: better to take the safe "draw" than to lose points.
2. Once you're at the top, you have no reason to continue proving your skill. You only have reason to not play.
3. The Glicko system tried to solve this by introducing sigma into the equation (Trueskill is Glicko-based), and I think Glicko is great for matchmaking, but other problems come up for ladder display. For example, many XBox players complain that if they play many countless games (decreasing sigma) but then genuinely improve rapidly, Trueskill is very slow to reflect this (and why wouldn't it? You've proven through many games that you're bad, so sigma is suspicious of sudden skill bumps, and rightly so).
In short, the SC2's hidden MMR, bonus pool/inflation/keep people proving their skill, division, and 100 player local competition ladder is absolutely brilliantly designed. I think I heard they hired on a PhD statistician for the task.
How about separate "Qualifier Ranking" and "Tournament Ranking" points that you only accrue by playing in sanctioned events? Qualifier ranking would be an opt-in system for those interested in their global rank. Tournament ranking would only apply to those actively playing in competitive events, to avoid the problem of highly "ranked" players who aren't part of the competitive community. Of course, these are just cosmetic community-facing bits.
As for the non-hardcore players, the genius of the bonus pool system is that it's a highly *visible* reward, but again, it doesn't actually help with the matchmaking. It's a psychological trick to encourage play, not measure performance. Of course, this raises the interesting idea that perhaps matchmaking shouldn't be based on "ranking" at all. Then you can award the match loser some experience points and everybody leaves happy, without worrying that it's messing with the matchmaking system.
Those intermediate (i.e. non-win/loss) metrics? Well, they can be used to "imperfectly" match people. Sure, there's a risk of people artificially manipulating those, but manipulating any one thing is probably to the detriment of something else, so it probably evens out over time.
I think carwashguy's explanation of why matchmaking ranking score should not be tied to displayed rank is pretty good. What he's saying is that my original claim that matchmaking ranking should be kept pristine is correct, and he gave reasons why humans actually would be happier with a different kind of thing displayed. One trouble here is that even if that is correct (and probably it is), it's a tough PR situation. The very people would would be happier with MMR working one way and displayed rank working another are the people who will SAY that they wouldn't be happy. They will claim that they are only happy if displayed ranking is "real" like the MMR.
The more I think about it, the more I think that it's ok to display a "not real" ranking and keep MMR pristine. It's ok to display a monkeyed-with rating because...the entire rating is fake to begin with as far as I'm concerned. While knowing a person's rank in a leaderboard does tell you more than 0 information, all that really matters to me is performance in open, non-invitational tournaments. If you are winning those, that matters and it's real. If you aren't, tough luck. Leaderboards are kind of a fakey thing to begin with, so using a leaderboard system that encourages more play and makes people generally feel good is probably a good idea because it grows the scene and keeps it going.
The very top division should still be handled differently so as to expose more about the real rankings, but tournament results are what matter at the end of the day anyway. At least imo.
Just because the official rating isn´t "accurate" - I really doubt that it´s possible to divide players into 3 groups, 1 Individual, everyone that is better than that individual and everyone that is worse than that individual - doesn´t mean it´s entirely fake. If it were Blizzard would simply shoot themselves in the foot.
I think that is also the idea behind the Divisions system. If you are nr.1 of Gold you don´t know if you are nr. 3 Million or mister 4 Million in the world BUT you might be able to say that you are even with the other Gold nr. 1, better than Silver and Bronze but not as good as Platinum. It´s much more motivating and accurate in the way that going from Gold to Platinum is a better representation of skill (at that level) than ladderank 4 Million to ladderrank 3 Million.
Outside of the very top 1% there is really little reason to be hyperaccurate, you don´t need or want such an competitive ranking for Players that aren´t competitive (yet). These Players are not exactly interested in even matches, and while even they don´t expect to always win they want :
1. not a complete onesided mismatch
2. recognize progress.
3. "Hope" of being capable of catching up. It´s also more of a mind trick but you don´t want to give Alice the impression that the missed Time is an eternal flaw that can only be made up for if Bob and Charly stop playing for the same amount of time.
In that regard ELO and Truesekill have no concept, they are developed for the Top 1% and applied to everyone.
Nice article. Much less annoyed at bonus points now that I've thought about it from this perspective. Also, I agree that it's a nice way to let people who aren't playing the game have their rating 'decay' as their skills are not improving/declining, but without the psychological effect of decreasing their rating.
Nitpick: The Elo rating system is named for its creator, Arpad Elo. Only the "e" is capitalized.
To me, what ultimately detemines whether a system works or not is "Does this multiplayer match up provide me with an enjoyable, balanced, play experience?". By the sounds of things, that's what the Blizzard system is trying to aim for: some sort of encouragement to keep people playing the online matches, but they aim to match skill as evenly as possible. If you want to decide the ultimate pecking order of who's the best, you hold qaulifiers then tournements which is what will really determine who is the best of the best.
My experiences with TrueSkill, however, are not always positive. For instance when I played Global Agenda, I was an above average sniper. But in effect, this would be penalized as teams would be set up to "punish" players who were having a hot streak recently, as opposed to keeping the skill levels fairly balanced. In other words, the 50/50 win loss was strongly enforced, where if you were moving towards 60/40 you would be put on a team full of glue sniffers to knock your win/loss down a peg. So in effect, the teams weren't arranged to have a closely competative fair match, but rather to "punish" people who were doing too well by pitting them with a useless team.
To me, this is no measure of skill, and is very fustrating. Even Fatality would get his ass handed to him if he was up against 10 professionals and his team composed of 9 scrubs who just got Quake 3 for christmass a week ago. But what does that prove? That Fatality cannot beat 10 pro league players alone, and that's about it.
In the above example of handicapping players by teaming them with unskilled teammates, it's terrible for you as an individual, but it's *theoretically* more enjoyable for all of the other participants in the match. Your unskilled teammates get an experienced player (you) to assist them, and your opponents encounter players with a variety of skill levels. Of course this doesn't work in practice; the low-skill players feel overwhelmed and the high-skill players feel handicapped. A "better" solution is to pair players together by skill level, and then assign them to opposing teams.
You should update your article about scrubs with a few paragraphs on Starcraft-Pro IdrA. He is the perfect example of how narrow minded scrubdom can even take place at a high (pro) level.
Agreed Alan, I can understand say myself and a couple other good people being pitted against 3 good people on the other team, and the rest being new players. The best games I've had in Global Agenda were the ones that went right down to the wire, and you had to dig deep to win, or barely lose. To me those are enjoyable play experiences, and even at a loss you can say you had a fun game.
The problem is 9/10ths of the games were two minute blow outs, where either you won the match too easily or got completely destroyed. If you did too many blow outs, the algorithm will put you on a team full of the worst low level players they can scrounge up and your opposition will be a Dream Team the best level 50 players from the best agencies. Having an empty easy victory or two only to be flat out annhilated in the next round by an unwinnable game (again, even Fatality couldn't pull a win with some of the glue sniffers I've been dumped with) isn't good design. Perhaps it could have been due to a lack of players, but after a while you get a disctinct feeling that the True Skill will "punish you" rather than trying to arrange fair games.
This article shows exactly how cynical Blizzard is towards their customers.
Actually Harunobu, maybe your post just shows how cynical you are?
When there are literally more than a million people on the leaderboard, showing your rank out of a 100 in a league is not such a completely crazy thing to do. At least it's some understandable information that doesn't vary wildly, like showing your straight up rank of 734,221 would. That rank could actually go down even if you won, because of all the volatility and variation of ranks changing around you.
It's also not so crazy to have a mechanic (bonus pool) that encourages people to play more. That means more opponents, better matchmaking, and a more vibrant scene. I'm still not sure if matchmaking uses an untouched Elo number underneath it all, but some people are saying it does.
While there could be other ways of presenting understandable ranks and encouraging people to play more, I don't think you reject their system out-of-hand as "cynical." It's supposed to be a system for humans to enjoy.
Why not trick everyone in believing they are no.1 out of the total million players? If that were possible, would it be a good thing? Do people really want that?
Half of SC2's ratings on Amazon are 1 star reviews. This is one of the reasons. The other reasons are created by the same cynical approach Blizzard takes. It is good for the stockholders, but that's a differen issue.
First time poster - but have been enjoying the articles for a while now, thankyou!
Just to say that I second Datawraith's opinion on the first page of comments - I think that ranking people by their location is an excellent way of dividing the players in a non-random way and if also used within the league tables by ability would keep the figures within your 'monkeysphere' (never heard that term before, love it!).
I don't like the idea of seasonal point wiping though as it seems a cheap fix to a nice progressive system, a more gradual way of limiting the numbers would I think be better as the more experienced players (in a similar fashion to an earlier post) wouldn't be as bothered by their score values but by their rankings. Is there some way of doing a daily/weekly/monthly deduction (based on ranking or percentile maybe), so that the worst players will lose almost nothing (fractions of a point) but the top players have to use all their bonus point allocation to keep their score at the same level? More carrot for the beginners, more stick for the pros kinda thing and also a way of eliminating the idle players from cluttering up the scene. I've only a cursory knowledge of the math in the Elo system and haven't even looked at how it differs in Trueranking so I'm not sure myself if this would work or not.
Sirlin,
Vanick and I put together a Part 2 of our Ladder Analysis thread, located here: http://www.teamliquid.net/forum/viewmessage.php?topic_id=142211
It attempts to go deeper into the mathematics of the system than the initial analysis.
We'd be interested to hear your thoughts on this, whether on your blog or TL.
Excalubur_Z: great, I will check that out soon. Thanks for letting me know.
Before I say what I think is a good idea or not, I'm kind of confused on the facts. You have a long section about ayesian inference, I get that part. Your promotion section, I think you are speculating about this? Or do you know that is actually how promotions work?
The displayed ranking thing is the biggest question. You're saying my MMR is compared to your displayed ranking + league threshold. That's kind of a crazy method, isn't it? And you sure it's really doing that? Seems like it would do a poor job in showing who's favored because the person actually favored is (by definition?) the person with the higher MMR, period. Showing "favored" to both people at once seems crazy, maybe that's just a bug or something?
I'm also not quite following how this same comparison (MMR vs rating + league threshold) would be used to compute the difference in displayed rank. I can sort of understand why it would be the way you said, but I would have guessed the MMR is not involved at all in ratings calculations. You have some basis to believe it is as you said though? I'm not claiming you're wrong, I just have no idea really.
The next biggest question I have is what the bonus pool points actually do. It seems you don't know either. I assumed they do count in all the calculations of your rating points going up or down, and you guess that they don't count. I guess no one knows yet?
Looks like complicated stuff to sort out before we can even form an opinion on if it's good.
Everything about promotion is speculation. Blizzard has literally zero information about how they work, other than there are "review checkpoints" and that the first one occurs around 30 games (we graphed this on the last page of the thread, a large spike of promotions occur around 22-26 games). However, are those checkpoints a function of games played or a stabilized sigma? There's not enough information to draw a conclusion (though we're trying to get more stats from SC2ranks.com's Shadowed).
As far as we can tell, and this is backed up by evidence from WoW Arena, the points (displayed rating) earned from a match are determined by comparing the opponent's MMR to your current point total. First, you are matched against a player with a similar MMR to yourself (say 1500 vs 1500). If you and your opponent both have 100 displayed rating, you will earn over 40 points for a win so that your displayed rating rapidly approaches your MMR, or you would lose 0 or 1 points for a loss. The points gained and lost are not zero sum. It's possible that the points earned or lost are compared to your own MMR, but we don't believe this makes as much sense because there needs to be some external comparison (your opponent has to be involved in the calculation somehow or it won't matter who you're playing). This mirrors the Favored message that appears in the loading screen, where you are playing against opponents that are Favored for a very long time. It feels like a bug to the player when both players see the other as Favored, but it makes sense in this context. It's still confusing though.
The bonus pool doubles your points for a win. It accumulates at the same rate for everyone at a rate of about 12 per day. Nothing is deducted from the bonus pool if you lose. This encourages players to play more when they have a bonus pool because even if you go 50-50 your point total is increasing for as long as you have it. It has the added effect of pushing inactive players further down the rankings (because everyone else will be using up their bonus pool with each win and climbing the ranks), but it gives them an opportunity to catch up again. The bonus pool total is determined from the start date of the ladder season, so someone who starts playing on Day 1 will have 100 bonus pool, but someone who starts playing on Day 11 will have 220 bonus pool. We believe the bonus pool isn't counted when calculating the points because it's a constant that only serves to inflate ratings.
i dont get the whole system... even after reading this. the enemy is always favourisied, i never was. and why cant i see other leagues ???
It's probably worth noting that Blizzard has an "official top 200 players" page now. It's not directly tied to the discussion of the points/rating and matchmaking systems, but there were some commenters who were wondering how the high end players could compare against one another since they're all at the top of various Diamond leagues.
http://us.battle.net/sc2/en/blog/600592