I've finished the single-player portion of Portal 2 last Friday and since it somewhat stayed with me the past two days I feel compelled to add my thoughts to this thread. There'll be spoilers, of course.
Well, I like it a lot. I don't think that was ever in question. Between Valve's track record and all the funny videos and news bits we got leading up to the release I'm pretty sure everyone who liked Portal was convinced that the sequel was going to be at least very good.
I could be cute and say that, under these circumstances, the game is about as bad as it possibly could have been. It probably makes more sense to say that Portal 2 is the least surprising game I've ever played. Although I guess that's a surprise in itself. Sure, there are a lot of quirky, interesting little touches, but well, those were expected, too, weren't they?
What Valve are doing when they are developing a game, and this really comes across best by listening to the developer's comments of the Orange Box, is to whittle away every possible frustration the player could encounter. Everything, from the lighting to the placement of the tiniest object in the game world seems to get honed to perfection, in order to guarantee the player a smooth ride. This could make for a very sterile experience, but Valve counter it by giving the "window dressing" as much character as possible. The writing, art design, music, sound effects, voice acting and especially the character models (Valve seems to be the only development company which likes interesting looking characters, as opposed to simply beautiful or ugly ones) – everything is unique and exciting.
Compare this approach to, say, how Bioware handled their big sequel, Mass Effect 2. Bioware is a much more uneven developer, not only because of the nature of their games. Take the inventory system from Mass Effect 1, for an extreme example. For my money, it's the worst I've ever seen. Something like this would never in a billion years make it into a Valve game.
The other side of the coin is that Bioware games are somehow more alive and vibrant. They aren't designed solely with the player in mind. And Bioware really tried to expand and improve upon Mass Effect 1 in a much bolder manner compared to what Valve did with the Portal franchise.
As I said, Valve counter the somewhat sterile gameplay of their titles by giving their games loads and loads of character. Only, in the case of Portal 2, it doesn't quite work, because it mimics it's predecessor to such a high degree. Of course I wanted GLaDOS back, too, but Portal 2 tells almost the same story (twice), has almost the same boss fight and, in the end, Chell finds herself in almost the same spot she was in at the end of Portal 1. Since the unpredictability and strangeness of the first game was such an important part of the experience, there's certainly something missing in the sequel. I was genuinely disappointed with the ending, which ripped off Portal 1 almost step by step, and as such didn't have the same impact. The ending of the first game was one of the only ones I've encountered that didn't just bring things to a close, but also added something valuable to the whole experience.
As for the puzzles themselves, they were fine, but left me somewhat wanting. Which was the case with the first game too, which gave me a feeling of, OK, now I understand the mechanics, now give me some really tough nuts to crack. Portal 1 had these advanced versions of some of its test chambers and a few genuinely interesting achievements (like "solve this chamber but use only x portals), which were very enjoyable. I understand that they wanted everyone to finish their game, I just hope we get some serious test chamber DLC for Portal 2.
I really liked the gels, they have a fun, experimental component about them. I liked other elements less, for example the jump pads. These catapult you to a very specific location, and it's almost impossible to change their trajectory. They even face Chell in a desired direction when using more than one in a row. I've noticed something similar with the weighted cubes, which always seem to activate buttons and stay there, even when I aim them poorly. It just feels less like physics (science?) and more like slotting a Tetris block into place.
The game is really funny, though. Where Portal 1 had only one funny character, the sequel has three and the writing and voice acting is of the highest quality throughout. And then there is the Turret Opera.
In the end, I feel somewhat like Kieron Gillen, who said somewhere in the genuinely thoughtful RPS verdict that he likes the game a lot, he just isn't sure if it's a classic.