While I still have it fresh in my mind I want to put down my thoughts about the single-player campaign in Portal 2. I haven't played any of the co-op, primarily because of the mysterious, ongoing problem with the PSN. However, I completed the solo campaign, and while I agree with many of the criticisms Michael Barnes made in his review at No High Scores, I greatly enjoyed Portal 2. I don't think it's a 10/10 masterpiece, but it is a very good game.
One reason I felt it didn't quite measure up to its predecessor is that the sense of discovery just isn't there anymore. Portal was about progressively seeing more of what lay behind its literal and figurative walls. It's almost impossible for a sequel to be as much of a revelation as an original story, but that's not all that's going on here. Portal 2's changes in setting aren't as tightly coupled to the story as in the original, much to the game's detriment. In particular, the journey through Aperture Science's past feels very much like a detour. As is often the case, the explication of the backstory felt a bit like navel-gazing, and as a result the overall arc just doesn't feel as immediate or vibrant as in Portal.
As for the new characters, Wheatley and Cave are funny, but they're far less original and interesting than GLaDOS. Cave in particular felt like J.K. Simmons slapped a southern accent onto his portrayal of Jonah Jameson and kept rolling. It's tremendously effective, and all the voice actors do phenomenal work (a scene where GLaDOS cheers on one of Cave's recorded rants is perfect). However, the new guys both feel relatively one-note in comparison to GLaDOS.
The natural expectation is that the new puzzle-solving tools (light bridges, light tunnels, paint, lasers) would open up the solutions, but with few exceptions they seem to do the opposite. For me, having to juggle the different kinds of tools made the puzzles feel more specific and constrained than they did in the original. I can't quite put my finger on it, but something about having just the one tool made the puzzle solving in Portal feel creative, even transgressive. In Portal 2 I felt much more like I was trying to guess the designer's solution than coming up with my own. The puzzles are still a ton of fun, but the actual process of solving them feels more like a test and less like play.
This may also reflect other problems with the puzzle design, which I felt was generally less robust in this game. In particular I found that the middle section of the game was difficult to read in ways that weren't particularly interesting. This part had very large spaces with exits that were sometimes difficult to locate, and figuring out where you were even trying to go became too large a part of solving these zones. The feeling of frustration was exacerbated by the fact that the solution was sometimes to just shoot a portal onto a surface that was far away and hard to see, giving some areas the feel of a pixel hunt.
If this sounds like quibbling, in a sense it is. It's a measure of the quality of Valve's work that being faced with a pixel hunt or two feels like an onerous burden rather than just part of the normal course of play. Portal 2 is great, and you should play it. It's not as great as Portal, but I can think of plenty of developers who would be overjoyed if "not as great as Portal" was the worst thing you could say about their game.