I'm almost certain I'm about to cover points I've gone over before, but if I'm still being bothered by it, it's worth harping on until it goes away. To paraphrase Mark Twain, if a man is dissatisfied it is his duty to harp on about it, even if he thinks he's the only one, just as it is the duty of every other man to vote him down if they disagree. And if they don't disagree, perhaps it's time we started getting some organized harping on going.
So anyway, sequels. Scourge of creativity. I've declared many rules in the past as to how sequels should be done - firstly that ideally they should be banned altogether, then, in my weak-ass conciliatory way, that was downgraded to a law that they should only be developed by people who didn't like the first one much. Later, in passing, I mentioned that a good sequel is one that sees the original as a jumping-off point, something to build on, rather than one that merely wallows in the original and "pays respect." Well, I'm going to add to this list of commandments now: You should never try to get a sequel out of a game that had a perfectly fine, self-contained story with no sequel hook, because it will never, ever end well. The only exception is when the sequel is almost entirely unrelated except for a few broad elements, as in Silent Hill 2, but when you're trying to squeeze another adventure out of the same characters it can't possibly have the same impact. Consider Bioshock 2 against Bioshock 1. God of War 2 against God of War 1. And indeed, Portal 2 against Portal 1.
Portal 1 was great because it was as long as it needed to be. The trick it pulled was that it appeared to simply be a fun little puzzle game with some funny instructional dialogue and only the bare minimum of plot required to connect the puzzles. Then, with only the merest dropping of hints throughout the game, the plot suddenly escalated towards the end. It was an interesting surprise and very well paced. The ending drew a firm line under it all, and while there was the overt implication that GlaDOS was still alive (hence the song), the player character escaped and there was nothing to be gained in victimizing them further.
In Portal 2 the plot is now center stage just in time for it to have very little more to say. All it could really do was paddle around for a while establishing backstory for things, and uncovering the mystery behind things can only ever make them less interesting. And the puzzles didn't feel central anymore. Form was defining function rather than the other way around. I never got the impression that any of the new puzzle mechanics had any kind of decent payoff, you just got to play around with them for a bit before moving onto the next bit of plot.