It was good. Dialogue was sharp, funnier than I expected. As a spectacle movie, it ends up feeling like a lightweight Transformers, never near as exciting as Michael Bay's trilogy (although also never near as obnoxious).
After the opening action sequence, in which Loki single-handedly wrecks S.H.I.E.L.D., I think Nick Fury says something along the lines of "This is a level 7," immediately prompting whispers in the audience about what would constitute a level 9 or 10. The situation does later escalate, but, even at the movie's biggest action sequences, "level 7" sounds about accurate by my scale. I was not even convinced that Thor by himself was ever quite having to give 100 percent. The cannon fodder aliens seemed no more dangerous than the entirely earthly threats of Iron Man 2. And Captain America was totally useless.
Mark Ruffalo was good as Bruce Banner, although, the way the Tony Stark character has been conceived as an uber-genius in these movies, one wonders why the team would ever need any other scientists. I can see that Hank Pym (AKA Ant-Man) would have been redundant here, but I still hope he and the Wasp are in the next one. And, no, he's not just a guy that turns small and talks to ants; he also has expertise in robotics and AI, and he created Ultron, one of the villains that I personally would most want to see as the heavy in an Avengers movie.