Captain America did his share too. He helped save innocents from being killed. Hawkeye also knocked Loki out of the air so he could get incapacited by the Hulk, and Black Widow did help shut down the portal.
Captain was also, well, the captain of the team. He (besides Thor) is really the only one there with experience leading a group into a battle, so his job besides saving civilians on the ground was to lead the Avengers.
So how would The Avengers compare to a great movie like The Dark Knight? That's the greatest superhero film I've ever seen.
Two different movies, I wouldn't compare the two.