Well, I sort of hate Portal 2.
1 - Puzzles were less satisfying and more aimed at giving cheap entertainment, though there was certainly some entertaining gameplay to be had.
2 - Plot was nonsensical and filled with holes.
3 - Wheatley was over-used until all the charm was drained out and didn't even work as a joke villain.
4 - Wheatley's character makes no sense. He was designed to be the ultimate idiot in order to keep GlaDOS harmless but he ended up being a far more intelligent and far greater threat than GLaDOS ever was.
5 - Aperture Science stopped being an enigmatic, interesting little coven of taboo science long abandoned by human thought, and instead of being a frightening machine of terror that had been left endlessly turned on without a purpose, it was some impossibly large freaking city-sized empire of science with no explanation.
6 - Attempts to explain the origins of Aperture resulted in a disappointingly vanilla story about Cave Johnson who isn't as funny as Saxton Hale.
7 - The worst part of the entire game is how they treated GLaDOS. GLaDOS was a mysterious and scary villain plotting whose goals are unknown to the player. Gradually as the player learned more about her we realized how out of touch she was as we observe her failed attempts to make us sympathize with a cube and we see her fall apart and panic when Chel slips behind the scenes of Aperture. We realize GLaDOS is just a computer that doesn't understand humans and is confined by her programming and her character arc is guided by the player's exploration of her. Eventually it turns out she is just a mad dog that needs to be put down and the depth to her character is perfectly exposited throughout the entire game.
In Portal 2 she is reduced to a joke. A non-threatening villain who's only purposes are to be mocked for the sake of vaudeville, to spit out already tired jokes and insults and to be shown up by Wheatley in order to raise the stakes. Finally she's turned into a bloody potato because potatoes are hilarious and why not twist the knife when taking apart everything that makes a character good?
8 - More on GLaDOS - Her origin is bullshit. It is revealed that she is based off of some woman named Caroline. This is a very interesting turn of events. Who was Caroline? How does this affect GLaDOS as a character? Not at all, you say? At first it seems as though Caroline is the humanity of GLaDOS that has been holding her back, perhaps GLaDOS was even self-sabotaging this entire time in order to avoid killing Chel. Nope. GLaDOS inexplicably "deletes" Caroline and still shows mercy to Chel. Seeing as we know nothing about who Caroline was before she was turned into GLaDOS and there are no differences to GLaDOS' character after finding out about Caroline and deleting her, I guess the entire origin was pointless. Yay!
9 - The humour. So, Portal was funny. Quite funny in fact. Well, let's capitalize on it and make Portal 2 REALLY funny. The difference is that Portal wasn't just funny, there was more to it than that. Aperture was a very interesting and unnerving place to discover, because of course you have no idea what's going on there or how you got there. All you have to guide you is the voice of GLaDOS, a character who's motivations are unclear to you. This is a great setup for exploration and enigma. Portal 2 has none of this. Wheatley, Cave Johnson, GLaDOS, Aperture and everything else is only there to make you laugh. There's nothing wrong with that, not everything has to have a thick plot, but GLaDOS' dialogue was so hilarious because you weren't expecting it. GLaDOS could have been anything, but you know that every character and plot twist in Portal 2 is there to make you laugh. They squeeze heavy-handed humour into every line of dialogue in lieu of the far more subtle approach that the first portal had.
The characters are less entertaining, the story line is meh, the puzzles are easier and shallower and there is generally less substance. Would I be kinder to Portal 2 if there was no Portal 1? Probably. However, I wouldn't have such a high opinion of Portal 2 as I do for Portal 1 if the latter never existed. Most of my animosity towards it isn't due to P2 being less good than it's predecessor, but rather due to the mistreating of a beloved character and setting. I believe that when it's looked back on, Portal 2 will be seen as little more than a footnote to it's predecessor.
Wow, I really needed to get that all off my chest. Thanks, internet.