The second half of the film is action, action, action. As it should be - you get these heavy-hitters together, I expect nothing less. My one complaint, without giving anything away (since it's mentioned in the IMDB plotline) concerns Loki teaming up with aliens. This isn't just apples and oranges, it's more like apples and oil filters. The worlds of Asgard are so rich, and full of villains - what's wrong with attacking Earth with frost giants, the Midgard Serpent, the legions of Hel or the dark elves of Svartalfheim? A war between Earth and one of the other 9 worlds, like Jotunheim or Niffleheim, would have been a more logical progression from the "Thor" film, and would have united the Avengers just as well. They can do aliens in the next film.
Second !