In all the sound and fury of the reaction to Portal 2, it is hard to have a more subtle critical dialog about the game. There's two main points worth exploring: which go in two different directions and which I have hinted to earlier.
1. The disparity between the overall critical response and the overall end user response. This is part of a wider conversation about the complexity of reviewing a game at release. I don't mean this needs to be a conversation about the ARG, in fact I'm not especially interested in it beyond watching the car crash of Valve's clever and inspired marketing and overwhelming end user expectation. If Portal 2 was an unqualified success, the end user grumpiness would be quickly silenced. But the fact is that the critical response is so overwhelmingly positive, when in fact there are problems with the game - such as the weaker story with some really ham-fisted moments and plenty of missed opportunities; that if you don't warm to the two-dimensional characters, Wheatley especially, you'll be left out in the cold; that you need to play co-op to experience puzzles which are actually challenging; the final encounter ends in a train wreck of cut scenes and music.
2. The feeling that Valve's game design somehow doesn't ring as true as it used to. I had the same problems with Left4Dead 2 as I do with Portal 2: for almost the whole game there is this feeling of one obvious path, which for me ended up me rushing through the levels waiting for the 'good bits' to begin. Paradoxically, it took me 9 hours to complete but almost all the delays were me missing switches and not using angled ramps for their intended purpose[1], which meant I would repeatedly try weird alternate solutions. The puzzle design in single player rarely involves any planning or experimentation: it's all spoilable in the sense that once you figure out the problem, implementing the solution is trivial. I particularly miss the sense of escalation to the 'you've got to be kidding me' level of complexity of the first Portal, something Valve has done well since Half-Life: instead here is all big is better, and 'distance fog makes levels awesome'.
The biggest flaw to me is the sense that games should be aspiring to something other than this. Maybe I'm over reading what is an encapsulated game experience, but the AI director and limited experiments with procedural level generation lead me to believe Valve was moving in a different direction. Portal 2 feels like a dinosaur raising it's head to watch the oncoming Minecraft block asteroid.
[1] Spoiler: Which is always jumping.
I am.