There's a lot of pressure on sequels and people have a lot of ready-made b.s. to unload. I understand the idea that the sequel has to live up to the first in some way--while also not simply recycling what worked about the first. But there's that attitude that the sequel is destined to be crap because it's just a naked cash grab--that makes very little sense. What was the first, just done for some altruistic reason other than cash?
That said, I went into Thor: The Dark World with no pre-made animus against sequels and franchises, and just a hope that it would continue the fun and interesting work done in the other Marvel movies. (What are we at now? This was the eighth movie in the Marvel Cinematic Universe. That's a lot of baggage to carry--though less once you realize that the second movie was Norton's Hulk movie.)
This being a professional work of craft, there are a lot of things that went perfectly right or that can't really be complained about. So: things are pretty, both real things--they had a credit for a leatherworker--and not real things, like dark elf spacecraft. And the acting is good. (Also, I saw someone complain that there wasn't a personal touch in the film, but except for Whedony dialogue in The Avengers and Shane Blacky dialogue in Iron Man 3, I'm not sure what sort of overt personal touch there is in many of the other movies. So that seems like a critique that hasn't quite had time to bake.)
That said, the story isn't very interesting, starting (as these things so often do) with the antagonist: Malekith. So the dark elves lived before the universe and want to plunge it back into nothingness. At the same time, the dark elves live on one of the nine realms that exists in the universe. I can get past weirdness like that (there's a way to reconcile that), but we still have a really big goal (destroy everything) with a very abstract drive (I am king of nothing). It's a very unengaging villain goal.
Which is probably why Thor 2 is set up so that much of Thor's conflict is with his dad, which is pretty ho-hum and a bit of a replay from the first movie. Sure, dad now approves of his son but disapproves of his son's love life (as opposed to the first, where he disapproved of both). And there's some argle-bargle about how Odin is acting so bad that he's as bad as Malekith. Wait, the guy who wants to protect his realm is as bad as the guy who would sacrifice everyone in order to end the universe?
Once again, the best parts of the movie center on Thor and Loki and Loki's confused status: he's a giant, but he's also an Asgardian; he's beloved by his mother but less so by his father; he wants power and respect, but what would he actually do if he had either?