It's not perfect, but I had a great time. Certainly a major improvement on the first (and I rather like that one.) It delivered on the epic sci-fi/fantasy action spectacle that I felt the first film was really lacking. Lots of great humor as well, especially from Loki. He really walks away with the movie. I hope the powers that be at Marvel realize what a gem they have with Hiddleston as Loki. They should continue to insert him anywhere it's practical to do so.
On the downside, Malekith is rather shallow as a villain. He doesn't have much to do other than be the threat. This is especially disappointing with Christopher Eccleston playing him. I really wish he had been more engaging.
As for the stinger, this one is problematic. Anybody without the requisite comic book knowledge is just going to be completely confused. It doesn't help that the thing is so awkwardly filmed, and doesn't mesh at all with the tone or style of the film that preceded it. So in the MCU the Tesseract and the Aether are Infinity Gems? I assume they're going to change form once applied to a certain gauntlet.