It has been discussed ad nauseum so I won’t go into how good it was, because it really was quite good. Can you sense the ‘but’ coming?
… the villains were terrible. I was never able to be completely sucked into the movie because there was never a point where I felt that Loki or the alien army (who I will call NotTheKree) represented a real threat. The movie actually acknowledges this a number of times, most notably with Tony Stark’s conversation with Loki in the penthouse and then later when the Hulk and Loki duke it out. As the movie unfolds, it is almost as if Loki was working with Nick Fury to bring this quirky band of lovable misfits together as a team rather than being a genuine super villain with a plausible plan to take over the world.
The dialogue was funny, the characterization of the heroes was enjoyable and the action sequences were big and comic-booky without being too incomprehensible, but given the choice I’d recommend Avengers: Earth’s Mightiest Heroes(Season 1 available on Netflix), the plotlines are stretched out over 26 episodes and it does a much, MUCH better job of making Loki (and other villians) a credible threat.