I may have realized this, but after thinking about it, there's another thing The Avengers made work perfectly, and it's the relation between the different avengers and Loki. Too many character but somehow they've managed to create scenes -that don't feel forced- for each of them:
Stark confronting Loki second before the final battle.
Black Widow in the interrogation room.
Hawkeye as his sidekick for most of the movie.
Loki giving Banner 'the look' and unleashing the Hulk (and then, smashing him)
A couple of scenes between Loki and, of course, his 'brother' Thor
Cap and Loki talking and fighting in Germany.
It may seem 'normal' when you first watch the movie, but it's a perfectly articulated script that uses every single character and makes them interact.