It’s all about the characters.
A well told story tells you a whole lot with very little. A few moments, a few lines, a few sketches of their history and hints at their future and the audience begins to believe that they know them.
In “The Avengers” not only does each character get his/her moment in the spotlight, but each one is distinct, defined and interesting.
Black Widow is in control (or is she?), Bruce Banner is a ticking time bomb, Captain America is everything good from the Greatest Generation and Iron Man is a punk-ass playboy, and each one says so… literally. So much characterization delivered in so few words is truly a thing of beauty.