Well, I listed my problems with Thor 2 a few times before, actually, but to re-iterate:
1. The washed out color palette made the movie drab. The first movie was kind of cheezy, but at least it was vibrant, bright, and colorful. I get that it's "The Dark World" but visually a lot of scenes were just dull.
2. The scene where the Asgardians are fighting those marauders was okay, but reminded me of the opening of Expendables 2: "Alright, action sequence, establish a Hemsworth as badass, and goodbye token Asian friend."
3. The villain was absurdly one-dimensional. I honestly don't know if Christopher Eccelston was tranquilized during filming or what. There was never any chance of being sympathetic or even remotely interested in Malekith. There was no melancholic discussion of the glorious of darkness, no rage at being driven into hiding by the Light, nothing at all, really. On top of that, he was a complete idiot - attempting to defeat your enemy by intentionally sacrificing 90% of your entire race, and failing? And then trying it again? It got to the point that, even if he won, it would be like him and three other Dark Elves left. Not really a whole lot of future there.
4. Odin was no longer the wise king of the gods, but just a complete asshole to everyone - Thor, Loki (okay, that one's justified), his own people, and especially...
5. Jane Foster was no longer a character, she was something that things happened to for most of the movie. There's a good 30 minute stretch of the movie where her character is technically present, but contributes NOTHING.
6. The weird retconning of Thor and Jane's relationship, from a life-changing meeting with the tantalizing possibility of romance, to "crying for weeks and staying in bed" - which completely counters what we see of her character in the first movie, in which she would have been driven to explore and understand the new information and possibilities that Thor had introduced her to, and that SHIELD's resources could permit her to work with. I mean this is the same woman who was willing to break a crazy homeless giant out of the hospital just because he might know something about a weird astrophysical phenomena she witnessed, and stand up to the Men In Black for interfering with her research. Yet somehow now, "OMG the guy I kissed *once* is gone CRY SLEEP CRY BINGE" - bullshit. It's more bullshit than Wolverine's obsession with Jean Grey, which is essentially carrying a torch for decades for a one-night-stand that didn't actually happen.
7. SO much expository dialogue.
8. How did Asgard become a galactic / universal power without figuring out "infantry should have ranged weapons"? I mean, for fucks sake, the Einherjar's swords could been capable of firing some kind of energy like a much weaker version of Odin's Spear or Mjolnir. Or at the least super-high tech throwing axes or spears even? Instead it was just getting shot to pieces until they got in melee range, which was just stupid.
9. The fighting scenes felt too small for the epic scale the movie was trying for.