I wrote about Portal 2's single player mode almost a year ago, here. I said it had great mechanics but was badly in need of some subtractive design, ideally by removing the middle 50% of the game.
I recently picked the game up again, this time playing on Mac rather than Xbox. I played through the entire coop campaign (meaning 2-player), including the "Art Therapy" extra chapter that was added after the game's release. I'm pretty blown away by the quality of the overall experience. Sometimes I give things a semi-joking A+ if I like them, but I think this might be a legitimate A+.
The coop mode doesn't have that long bad middle part that the single player game has, so that means it's just the good stuff left. It's also interesting to see all the puzzles that involve two players (and four portals), as that adds another layer of mind-bendingness to it all. You can do more things with 4 portals, and that means there are even more clever things going on in the level design to make things challenging.
Last year I criticized the single player mode for the suble reason that it seemed to be less about communicating an interesting idea (about whatever new interaction is being showin in that level) than it should have been. Meanwhile, the coop mode seems to have had the design goal of "make the players feel like they are really cooperating." That's a difficult goal, but the game is one of the best examples of coop play that I know. Even the tutorial at the very beginning has you having to give boxes to each other, having to press buttons for the other person to progress, and having you learn gestures to wave at each other or to point to things in the level for the other player to see. It's very much about "helping your friend." The juxtaposition of that design concept with the cold logical nature of puzzle solving makes for quite a mix.
The cooperative nature of the levels runs a little deeper than just having to hand a box to somone, though. In many places, the level design is such that a set of force fields (you can't shoot portals across those) separates one part of the level from another. One player will natural try to figure out what to do on one side of the force field while the other player will try to figure out what to do on the other side. This could very easily be a non-cooperative, 2-player solitaire type thing, but somehow it isn't like that. You can usually see what the other person is doing, and even when you can't, there is AMAZING feature of pressing tab to bring a picture-in-picture view looking out of your friend's eyes. So you can see what they are doing and suggest ideas of what might work. Also, in situations like this you really need an overall understanding of the whole room, not just one half of it. So you have to realize "if only you could get that box in your area over here somehow? Or if had a way of getting one portal on one side of your force field and your other portal on your side somehow, we'd be able to do X thing." There's a lot of planning you are encouraged to do together due to the nature of levels. Valve's A-team was really on this coop mode.
The difficulty of the game seems about right "for mass market consumption," as my friend put it. Kind of hard sometimes, but it never felt like something I couldn't figure out. While I think the difficulty tuning of the puzzle is done well, I do renew my objection from last year that it's unfortunate that the game's overall structure prevents Valve from designing any truly hard puzzles. I wanted there to be some really, really, really challenging puzzles, but that would require them to be something you could skip and still progress past, then come back to later.
For higher difficulty, I looked to the player-created levels. I discovered a map maker named Mathey2009. This guy's work is very, very good. It's ridiculous to see how few ratings his maps have and low rated they are (2.5 out of 5 stars? Give me a break!). Ignore those ratings, they are unjust and mysterious. I recommend you try his map "Cubelessness" first, and when you discover the "interesting use of the game engine," you should know it's not a bug. The map Suspended Animation is also interesting and fleshed out well. His map "The most challenging test chamber in the multiverse" is quite a beast, and intentionally pretty confusing. Finally, "Logic" is his hardest map. Probably it's so low rated because almost no one has finished it. (I finished all those maps, by the way! woot.)
Those maps are all single player. If someone can recommend some good player-created coop maps, that would be great. I have not played any other than Valve's, though I would.
Portal 2 actually has two different level editors. One is a simplified editor, now build into the game itself. The other is the full-featured editor called Hammer (for all Source engine games) that only runs on Windows, so who cares about it (heh). Hopefully they'll make a Mac version of that at some point. Anyway my point here is about the simplified editor inside the game that works on both Mac and PC. It's honestly amazing how simple and elegant the UI for this editor is. Making a Portal 2 level sounds like something only for super geniuses who aren't you, but actually you really can make a map very easily. You can resize the room or parts of it, rotate the camera view, add elements, connect elements, all with just clicking and dragging. No code needed, no numbers, no semi-colons or anything. It's slick.
Making a *good* level would be a different thing entirely from just making *a* level. Because of the nature of Portal 2 as a mindbending puzzle game, it's extremely difficult to make a good level. You have to care about it being solvable without forced-death, about preventing unwinnable states from being possible (dear Mathey2009: you need to work on that one, there are unwinnable states in Cubelessness and The Most Challenging Test Map), and you have to care about a hundred different "exploits" people might use to cheat past the actual difficulty of your map. It's damn hard stuff. But I'm really impressed that if you want to make *a* map, some kind of map that does something, it's easier to do than I would have even imagined because of the well-designed interface of the in-game editor.
Note to map makers: you can't make coop maps in the in-game editor, and there are many other limitations as well. But you can export the maps you make in the in-game editor into Hammer. So you could get pretty far using the awesome UI, then polish it up even more in the full-featured editor (on Windows), if you wanted.
My last thought is that it makes me kind of sad that if somoene made the best Portal 2 map in the entire world, that would be worth about $0. That's too bad! Somehow we gladly pay for stuff a lot worse than the best Portal 2 map in the world.