Most of the problems most people are currently having don't seem to make much sense to me... I doubt the game will be centered more around co-op than single player, the stuff I have seen about it is generally saying while co-op will have some more difficult puzzles, single player is still going to be very strong/atmospheric/funny/generally good. I agree that the length could be an issue, however with all the shiny new game mechanics to play with, I'm guessing it wont be a huge problem. Personally, the thing that worries me most about Portal 2 the most at the moment is actually Wheatley...
While he could be a good, humorous addition, part of me thinks that he might just end up getting in the way of things. With the game now apparently having 13000 lines of dialogue (Although half of that will probably be different tones of the turrets saying 'Hello'), I get the feeling I might end up saying something along the lines of 'Shut up and let me do my physics puzzle in peace'.
For me, and I assume several others as well, one part of Portal 1 that made it great was the atmosphere, & I can't see said atmosphere being improved if you have a robot following you for half the game that wont shut his mouth...
Either way, as long as the physics puzzles work and don't get tedious/boring, Portal 2 will still be a very good game... I think I'm probably nit picking, and really, this is Valve we are talking about... They aren't exactly stupid