Infinity Blade was the #1 paid app in the App Store a few days, and deservedly so. As a player, I have to give it a grade of A because it's entertained me more than most games do these days, including games costing literally ten times as much. I'd rather give you opinion as a game designer than a player though, and from that standpoint, I rate it even higher. What's most notable is how much the designers did with so little. A simple combat mechanic and a small map go a long way.
Infinity Blade is "Punch-Out with swords, combined with stat optimization and a hidden object game." Skip the next couple sections if you already know how it works.
Description of Combat
The basic mechanic of slashing with your sword has a good visceral feel on the iPad's touch screen. The combat itself is kind of like Punch-Out. You can dodge the enemy's attacks and after you dodge a big one, you have a chance to hit back. You can also sneak in other hits before dodging the big one, but they do less damage. You can block so that no timing of dodge is needed, but the number of blocks is limited by how strong your shield is. You get an XP bonus if you don't break your shield, so it seems best not to use it. Parrying is more fun than dodging or blocking, and it caries higher risk and a bit higher reward. To parry, you must slash at the right time and angle to match the enemy's attack. Very cool.
You also have two meters, kind of like super meters in a fighting game (simply touch them to activate when they are full). The one in the upper left always stuns the enemy, letting you hit them a few times for free. The one in the upper right lets you do a magic attack (based on which ring you have equipped) that requires you to do a gesture to cast.
Description of Stat-Game
As usual, you defeat enemies to get money and items, which make you stronger, which let you beat more enemies. The not-usual part is that the items themselves are what level up, and your character's XP only goes up as the items gain XP. Once an item has its max XP, it no longer contributes to your character's XP when you defeat enemies. The dynamic that results from that mechanic is that you want to constantly swap your items around, making sure you have decent stats, and also a decent XP gain rate. This might sound annoying on paper, but giving you something to manage between fights is, I think, a big reason why the game doesn't get monotonous nearly as quickly as you'd expect. You get to go back and forth between combat and stat-optimization often, and each gives you a break from the other.
Movement
This is really one of the main ponts I want to make. You cannot walk around freely in Infinity Blade, you can only click on the note you want to go next, and a canned animation takes you there. On paper, this sounds bad. Really though, I think it was a very smart decision. Part of the reason they did this, I assume, is for the technical reason of ensuring that the environment always looks good. The game is pushing iOS graphics further than any other game so far, and if had to have every nook and cranny have a high res texture when you walked right up to it...well that's probably just not going to work. But there's more to it than that.
Many years ago, the PlayStation2 games Urban Reign and Spikeout came out at about the same time. They are both "fight a bunch of guys" games (Spikeout is not a volleyball game, fyi). I think it's pretty clear that Urban Reign had a much better combat system. It even had a Guilty Gear-style burst! But Spikeout allowed you to walk from one battle to the next, while Urban Reign's combat took place in what was basically a box, then a loading screen, some story text, then another box. You didn't actually smoothly transition from one fight to the next. I looked on fighting game forums to see players discussing the games, and to my surprise at the time, more of them preferred Spikeout. Being fighting game players, I think most of them agreed that Urban Reign's combat was better, but "there's just something that feels nice about Spikeout in comparison," I saw again and again.
I think Infinity Blade captures that same feeling of travelling from fight to fight, but it doesn't bother with making you actually manually walk there. It probably wouldn't feel great to navigate a 3D world between fights (especially on an iPad or iPhone), so they cut straight to the point and have you automatically walk there when you tap the next node. This whole thing is subtle, and not something you probably think much about, but I think it contributes to the overall experience quite a bit.
StarCraft 2's single player campaign is a somewhat similar example. You don't actually see an animated walk sequence of your character going from the bridge to the lab and so on there, but the point is that you don't have a fully manual, free-roaming world there either. In both cases, the free roaming would add nothing so it's not even there and the "click to go there" method is used instead.
Fast Forwarding
A common tension between artists and designers is how fast things are. Artists tend to like elaborate and awesome animations, and designers don't want to slow players down with that stuff. We were just talking about walking from fight to fight, and that could get real old, real fast. Especially if the artists want a long shot, then a slow and dramatic walk with a closer shot. Also, artists are going to want awesome cutscene animations of finishing off your opponents when each fight is over. More annoying waiting for players?
Infinity Blade has all the goodness that art side would want (great finishing animations, dramatic walk animations with multiple camera animations) and with none of the downside. There's a fast forward button in the lower right corner of the screen, and it's really responsive. I expected it to be a "skip" button, but I'm much happier with the fast-forward button. It's very responsive in that you can fast forward for a moment, then let go, then keep fast forwarding, etc. It's totally up to you how much of the various awesome animations you want to watch.
Hidden Objects
As you progress through the castle, there are various bags of money and health potions laying around. There's an achievement for finding all of them on a given run through (I never got it). I soon realized that they are playing a bit of a trick on us in that some of the bags of money are actually accessible during those cut scenes. Yeah, those things you were fast forwarding through, if you actually let them play and scrutinize them, there's money bags you can pick up. I was kind of surprised to see that in addition to the combat game and stat-optimization game, there's also a hidden object game. Pretty cool.
Difficulty Progression
The castle is pretty small and you don't face that many enemies until you get to the end. So how in the world are they going to make hours and hours of gameplay out of that? The clever answer is that at the end of the castle, you fight the powerful God King. The first time through, he is crazily more powerful than you, so you are highly likely to lose to him (thought it is theoretically possible to win). If you lose, you restart the entire game on "bloodline 2" which takes place about 20 years later. You son (wearing all your same armor and mysteriously with your same stats) tries again. There is a kind of dynamic difficulty setting there, in that after more and more runs, you're getting more and more powerful, so it's easier and easier to take on the God King.
When you beat the God King, you get the obligatory cut-scene ending ("now they'll come after YOU"), and you still start the game over (now as the son of the you who beat the God King, apparently?). Anyway the real point is that now all the monsters in the castle are much harder. And if you beat the God King again, all the monsters are much much harder, and so on. This is a great way to get a lot of mileage out of such a small landscape and small number of fights per run.
Suggestions
I wish more information was presented to the player. Items have various icons such as "lightning bolt icon +8" but no explanation of what that means. A certain ring allows you to cast a spell called "Shock 3." Does that spell get more powerful with the +lighting bolt stat? If yes, the spell should have the lightning bolt icon next to it. If no, that's kind of confusing too. Another confusing thing about stats is that your character profile screen lists the bonus effect of your helmet (for example: item++, which means items are more likely to drop) but does not list the bonus effect of other items. This implies that you don't get bonus effects from non-helmet items, which is obviously stupid because why do non-helmet items even have them? Surely the profile screen is just failing to list all the bonuses, but it should.
The toucharcade.com review mentions that when you master an item (max out that item's XP), if you equip it, you get an extra bonus. This appears to be false (you only get the same bonus as you got the entire time you were using it while leveling it up), but it's an interesting idea anyway because you aren't getting XP from that item while wearing it anyway. I could go either way on that, but it's worth thinking about.
Infinity Blade also lets you do 3, 4, and 5 hit combos. I'm not sure if it's intentional or that the developers just didn't get to it, but for some reason there is no explanation of how to do them. Yes I understand there is some exploratory aspect in figuring it out yourself. When I weigh the good of that versus the bad, it comes up way bad in my book. I'd like to know the possible combos, and I'd like more of them. For reference, play Bayonetta and take 0.1% of their combos and put them in the next update for Infinity Blade, please.
The flipside of that, is that maybe this "Punch-Out with swords game" should actually be MORE like Punch-Out. I'm referring to the gimmicky nature of each fighter having some pseudo-puzzle to figure out. Maybe not quite as Punch-Outy as Punch-Out, but if you turned the knob a bit that way, I think it would add more variety to the fights.
Multiplayer
The next update for Infinity Blade will apparently have multiplayer. This will be a great opportunity for the game go completely off the rails. It does so many things right, I would hate to see the possibly inevitable decision here: multiplayer fights are based on how much time you spent grinding equipment instead of fair fights. This is an inherent issue in stat-based games, and it's a solved problem, too. The solution is to offer two modes: 1) a fair mode where you pick your equipment, and your loadout is kind of like your customized Magic: the Gathering deck. Everyone has full access to all options. And then 2) unfair fights using whatever you happened to acquire in single player. This way, the people who have fun by lording advantage over those with worse stats will be able to, while true competitors avoid any possibility of such nonsense.
That said, I'm very much looking forward to the update, more for the single player content than the multiplayer, actually.