The co-op campaign is amazing fun. Easy GOTY material.
I don't see that happening.
A. sequels can never truly recapture the same charm as the original, as much improved as this game is, it simply isn't a brand new experience, there are a few more tricks here and there but its ultimately just a sequel.
B. Just like Portal 1, I really don't see the replay value here. Single player is a good deal longer/more complex than the original, but its ultimately not that much longer gameplay-wise, again because it isn't original - puzzles that utterly stumped us in Portal 1 are easy peasy here and single player shouldn't take any ardent Portal 1 vet more than 6-8 hours to complete.
Heck, I just finished it in a little over 6.5 hours and I felt like I took my time looking for easter-eggs and trying to do achievements. And outside of farming achievements I really can't see myself running through single player again any time soon, any easter-eggs I missed will surely be on you-tube soon enough.
C. so while the single player feels decently long enough, it definitely is what I'd say is worthy of a $50 game, especially when the puzzles are so easy at least up until a point where they become nearly impossible out of the blue. Don't get me wrong, I say impossible for effect, I still managed to figure out the puzzles and keep moving to beat them game in relatively short order. However its all about the flow, I'm moving along and moving along and then all of the sudden I spend a half an hour trying to figure out a puzzle that has to be solved in such a particular way that if you mess it up its a PITA to do everything over even if it is only a couple extra minutes or even seconds (the game could have used a sprint function). This problem also occured in Mirror's Edge where anytime you have to stop and figure out how to get from one part of that game to another was kind of a drag because you just wanted to keep moving.
I dunno, I guess I'm just a bit disappointed because it really just felt like a far more polished Portal 1 with the same kind of puzzles but with different elements as opposed to having more puzzles that could be solved in a mutlitude of ways. I was hoping for a more open world that could be explored more instead of being as on-rails as it was.
D. However co-op is where its at, that's for certain. The most unique/fun/satisfying experiences from this game are from playing co-op.
But the only chance this game has for being considered for GOTY is just how long co-op is. I have yet to complete it, but I'm pretty sure my buddy and I were already just over half way through it before he had to go to bed, which would be disappointing.
Inferring that to be the case, or that its at least no longer than single player, I think we'd definitely need to see new test chamber maps on a regular basis, whether official or usermade (and if they're usermade they need to be very easily implemented). I'll definitely run through the co-op maps a couple of times with different buddies (if only to try and grief each other by trying to get the other killed at unexpected moments) but outside of that I don't see a point to keep playing over and over.
Sure, you can try and beat best times, but that simply doesn't match the thrill of live opponents where no 2 play-throughs will feel exactly the same.
Conclusion: Don't get me wrong, so far I have really enjoyed the game and fully expect to get more than my money's worth. I just don't think its anywhere near GOTY material from what I have seen so far (played through single player, half of co-op). I simply don't see it having any chance at all of beating any of the other anticipated blockbusters like Battlefield 3... or The Old Republic... or Skyrim... or Duke Nukem Forever... or Rage... or Brink... or Deus Ex: Human Revolution... or Mass Effect 3... etc...