I read this topic and remembered I had a copy of Portal 2 lying around my hard drive, so I fired it up and played for a bit. I must say I was somewhat disappointed by the linearity of the game. Endlessly I found myself shooting at stuff and going "aw can't put portal there". Eventually I understood that I could only place portals of the white surfaces, but then I got bored of the puzzles eventually. And I'm not the brightest at this kind of problem solving, so I had to bring up the walkthrough more times than I would have liked after trying for over half an hour on some levels, and the solutions made me feel like I was mentally challenged - I don't know if I should consider this a plus or a minus, to be fair. Perhaps both.

Maybe it came from my initial expectation of the game. I knew it had to do with portals seamlessly integrated into a 3D world, but the limitations on where you can actually place the portals significantly damaged the replayability of each puzzle and the degrees of freedom available to the solution. I was expecting a mind-bending, portals-everywhere sort of game where you can use your imagination to creatively solve seemingly impossible challenges, and instead I got an on-rails, one-puzzle-one-solution linear game...

Of course I've only played the first couple of chapters, I heard it gets better later on with enemies are more diverse levels, so I will try and make it there, but that's my initial appreciation of the game. I just feel the portal dimension was a bit underutilized (but who am I to criticize, they probably tried to make them more general-purpose and found this was the best compromise, I don't know).

It would be pretty hard to create any meaningful puzzles if you could put portals anywhere....

As I see it, it's often the limitations on what you can do that makes the game.

That said, I agree that puzzle games in general have limited replay-ability after you have solved the puzzle.

Portal 2 was done very very well, and I felt it was a worthy successor of the original.

Once you've beat the game, and played a few of the community made maps, there's only one thing left to do: Coerce someone to play Portal 2 coop with you. The coop is it's own (small) campaign with even harder puzzles since the two of you have to work together to solve them using 4 portals instead of 2.

The humor in the coop campaign (on occasion - not enough IMO) uses the fact that you're both playing on different computers, to hilarious results with things like Glados speaking to just one player when the other can't hear, trying to convince the player that the other one is trying to kill him.
The coop isn't as long as the single player campaign though, and ends kinda suddenly and luke-warmly. Still, about 8-10 hours of new maps with new jokes, using the new bot player characters, and 4-portal puzzles.

Also, for one more Wheatly-based touch of humor: ([size=2]Spoilers if you didn't beat Portal 2's single player campaign)

- Wheatly nominated for best character in a video game

- Wheatly's (unused) acceptance speech (Valve didn't get the award, but had the video prepared ahead of time)