The good- Chris Hemsworth is very well cast, and seems to have grown into the role quite a bit. This time around, his Thor is much more natural and human. And his interactions with Loki are excellent-- the brotherly chemistry between them really works.
Thankfully, the script for this sequel has a lot more humor than the original "Thor"-- I think the folks at Marvel and Disney learned something of real value when Joss Whedon's "The Avengers" was such a huge success in large part due to a script generously peppered with wit and humor. "Thor" benefits greatly from doing the same.
I think the entire cast from the first movie returned for this film, including Thor's Asgardian buddies and family, and they have some fun moments. Natalie Portman's Jane Foster is back and somewhat more interesting this time, and Kat Dennings as her assistant Darcy is good as well, but it's Stellan Skarsgard who steals every scene he is in. I love the continuity between both "Thor" and "The Avengers" and his storyline in this film.
The bad- Some scenes seem rather rushed-- there were a handful of quick cut-aways that felt abrupt. Really, that's my only critical comment. Well, maybe also that the "Dark Elves" didn't really seem all that scary. I never felt like the heroes were ever in real peril, but in a film like this, that's okay.
The awesome- The special effects and CGI are, as usual, phenomenal in this movie. As I mentioned above, humor also improves on the first film so much-- no comic book-based movie should take itself too seriously, and "Dark World" achieves an excellent balance.
But what really lifts this movie is Tom Hiddleston as Loki. He endears the character with such depth-- Loki is a trickster, he is conniving, wicked, and clever. But he also feels sadness and disappointment and frustration and Hiddleston manages to relate a wonderful mixture of emotions beneath Loki's veneer of confidence and sly trickery. He really is utterly compelling as the villain and his performance threatens to overshadow all the heroes of the Marvel universe. I look forward to seeing what Loki will do next. (Though, it's been said that he won't appear in the sequel to "The Avengers".)
Oh, and one more awesome is a cameo by... well... maybe I won't tell you. You'll see. It's adorable and funny and made me so very excited for the next few Marvel projects.
If you haven't made it to the theater for "Thor" yet, it's an excellent date movie, or take your teenagers. This is a great "popcorn" movie-- this film is what you go to the movies for. Fast, fun action and entertainment.