This film works because the plot is simple (effectively built around three clear set pieces) which allows it to be populated with lots of characters and gives them space to individually achieve something without the whole thing feeling overstuffed. He first Avengers film feels like the culminatation of something, unlike the sausage machine of current Marvel output.
Whedon has done a tremendous job here and deserves real credit. He clearly relishes the near impossible task of balancing focus between the individuals and the group while bringing to the affair his customary flair for sharp dialogue alongside impressive action set pieces. Although I understand how some hate what the genre represents, I think this is an exceptionally well judged concoction of fun and thrills.