Embraces it's superhero origins and becomes an entertaining superhero team-up full of action and great character dialogue. Hulk gets a redemption in this film and Scarlett Johansson has a more active role and totally overshadow Catwoman from that other superhero movie. How is they ever going to top this?
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.