Let me say, first off, that I think that aside from the somewhat-disconnected Hulk movie(s), I think that Thor is the weakest character in the Avengers franchise (the new casting for the Hulk in the Avengers movie pretty much resurrected that character from the "worst" slot). My son loves Thor, and loves both the movies, but Hemsworth is so leaden as the lead character that it is no wonder they gave Hiddleston as much rope as he could handle.
I get that Hemsworth looks like Thor should look, and that Thor the comic book character is pretty wooden on his own. But setting him aside, Natalie Portman continues to such the wind out of every scene she's in to the point where you really are rooting for Thor to hook up with Sif instead. Stellan Skarsgard is played strictly for slapstick laughs. Kat Dennings is also a comic relief reprisal, but I think she should have been cast as Hel rather than jokey-sidekick to the sidekick. And Christopher Eccleston has the double ignominy of being covered in enough makeup to be unrecognizable aside from his nose, and then having his voice tampered with to sound less human and more sinister. Making matters worse, he's a two-dimensional villain with zero personality who is just trying to wipe out the universe.
So, them's the bad points. The thing that isn't quite such a negative as much as a weird quality to this movie is that, much like the first one, it really isn't much of a superhero movie, at least not in the traditional sense. There's no plot element involving secret or alternate identities, no real theme involving a hero rising up to overcome some challenge. Malekith shows up, trashes Asgard in search of the Ultimate Power McGuffin, does some damage, Thor and Loki team up to stop him, and the final battle ends up on Earth, in England of all places (still no reason why they set the story there, and not in America, except to possibly explain why Captain America and Iron Man didn't show up to help). There's really nothing in terms of character growth going on here, just mostly a lot of CGI-heavy combat, which is visually fun.
Okay, I've ragged on this movie enough. It's better than the first Thor movie. There's lot of cool explosions, and it is part of the Avengers franchise. There's the part that is clearly working its way up to the next movie, especially after the credits.
On the upside, seeing the new Captain America trailer was cool, and the whole thing got me thinking about supers RPG's again, which I had been a little off of lately.