My impressions of Portal 2 so far (no spoilers).
The first five minutes are very promising indeed. The 'art appreciation' and the other interactions you're asked to perform are inventive and provide context for the story - even if some of them are a touch predictable.
The rest of the game is... less of a jump forward in narrative than I was hoping. Gameplay? Sure: there's more of it, it's more complicated, tick that box. But the original Portal held some of the sharpest, funniest, darkest writing we've seen in any game... well, ever, and I wanted that format to be built on.
The reality is that this game (up to where I am, what I'd guess is at least 2/3 though) uses that exact same structure: enter a test chamber, listen to your handler (in the first game, GlaDOS, here a number of characters), complete test, listen to handler, head to next test. In short, the Portal series already had strong character nailed down - I wanted to see that level of intelligence applied to a plot.
Sure, there's more going on here, but not a whole lot. You have two overarching objectives throughout the game, and they're basically the same one - the rest of the time it's finding excuses to put you in test chambers. You learn a lot about the facility, but nothing really surprising or meaty. The environmental clues that made up the bulk of the plotting in the first game return, but they worked better in the more minimalist original whose three hour playtime made the slightest sign of life or chance for escape seem big and important.
I wanted Portal 2 to affect me in ways beyond laughing. I wanted to be told a story, or to be taught a lesson, or to feel the drama. The heavy emphasis on comedy, and the fact characters are almost always talking about things entirely unrelated to the plot relegates their role to flavour rather than content.
Now, I've not finished yet. And I'm not saying this is not a GREAT game. It is. It remains superbly written, polished, intelligent, and bigger. But for me, Valve are refusing to push the bar in the one area I really want to see them try.