I've seen The Avengers and although the other movies listed above are not needed to enjoy it, they will GREATLY enhance your "OMG" and "Oh, WOW!" of the movie.
And I loved The Avengers so much, I just can't stand it. I will probably see it two more times in theatres. The character development is quite crafty and well done such that you get to know each Avenger but not at the expense of the overall movie.