Co-op, then. As the Laurel and Hardy-inspired droids ATLAS and P-body, it's your job to complete cooperative test chambers under the watchful eye of fellow AI (and creator), GLaDOS. She explains that the idea behind these tests is to observe how well a pair can work together to tackle a shared problem, and she dishes out Science Collaboration points based on your successes.
A running joke throughout the game finds you rewarded with points for successful actions, and penalised for botched jobs - though it soon becomes apparent that the entire system is yet another flourish of the wonderful script. GLaDOS revels in the competition between the two players, dishing out insults to anybody she perceives to be slacking, and praising the other in order to incite jealousy. While there's no narrative to tie everything together as there is in the single player campaign, GLaDOS' commentary on your activities is as amusing as ever.
With each puzzle demanding increasingly complex solutions, Valve has had to implement a means of allowing players to convey their strategies. A Ping tool lets players paint a marker on a surface or object, suggesting that their partner should whack a portal on it. Anybody attempting to play the game without a headset will find this indispensable, but those playing in the same room (the game supports split screen play) will find the tool equally as helpful. It might be simple, but sheer genius in terms of how it permits the simple communication of ideas.
The co-op half of the game is considerably harder than the single player, as far as I'm concerned, which makes sense given the whole "two heads are better than one" shtick. Being stuck on a puzzle was never something that bothered me, however. The process of discussing potential solutions with your partner and trying out different strategies turns out to be the most enjoyable thing about co-op play.
Valve knows how gratifying it is to nail a tough puzzle, and prompts players to celebrate their intellectual prowess in game via a gesture system. Pressing up on the D-Pad will bring up your gesture wheel, where you can choose to wave at each other, high-five, dance and even play Rock, Paper, Scissors. It might seem a trivial addition, but - on top of allowing players to interact with one another and celebrate - it really helps bring the two droids to life. I suspect many people will find ATLAS and P-body to be the biggest stars of Portal 2.
Unlike more traditional co-op, you're not just deciding who shoots which alien with what gun; you're carefully planning and executing a very specific strategy, an impossible task unless both players are doing precisely the right thing. It's rare that a game brings together two players in such a way, which makes Portal 2's co-op one of the best multiplayer experiences I've ever played.
I think you knew all along this was going to be great. Fantastic, even. All the same, I don't think you'll be prepared for it being this good. I may have buttoned my lip in terms of concrete spoilers, but a mere ten minutes here will yield more memorable moments than you're likely to squeeze out of the entirety of most other games this year. In all aspects of its design, Portal 2 is genius.