Saw it on tuesday. Here's what I thought:
Thor: The Dark World is both the funniest and the best-looking Marvel movie yet! I liked the first Thor quite a bit, but this one easily blows it out of the water, giving even The Avengers a run for its money in terms of entertainment value. The visual effects, cinematography, and production design are nothing short of breathtaking, the characters are hugely enjoyable all around, and the action-packed story, which effectively interweaves sci-fi and fantasy elements, is exciting, funny, and emotionally engaging.
If I had any complaint, it's that the villain isn't all that interesting, but this doesn't really matter much, since he doesn't take up a lot of space, and the interactions between Loki, Thor, Jane and the rest more than make up for it.