It sounds like you just expected the wrong thing. The Nolan Batman films are gritty, realistic films. The Avengers was more of a classic comic book style with colorful, larger than life heroes. Serious was not really the way to go with this one, and it wouldn't have made any sense given the established films in the franchise already. In fact, some actual serious scenes were cut from the movie because they didn't fit, even though they weren't bad scenes on their own.
--
http://img.imgcake.com/newaccountjpget.jpg