Robert Downey Jr. owns this film, but that is not his fault or the fault of the other actors. Compared to Tony Stark's suave and focused Iron Man, Marvel's poor superhero Captain America has little to do in a film where brotherhood should rule the day. For all the other characters, it's more of the same. Much ado about nothing.
Captain America's heart is full of integrity, straightforwardness, and outright old fashioned patriotism. Combined with his self sacrificial attitude and military training, Steve Rogers is a true hero and a nice counterbalance to Stark's cynicism and irreverence. It's part of what made his first appearance in his own movie so compelling. Chris Evans does the best he can with a script strong on visuals but weak on dialogue, terrific one liners aside. It's a problem all the others seem to share.
The centerpiece villain is left to inactivity for most of the film. Huge misstep. Tom Hiddleston is made to play the evil character, Loki, yet the script treats him as if he were a nuisance versus a real threat. His brother, Thor, is better here as part of the team than a solo star in his own film. Perhaps Chris Hemsworth just needed a better cast to pull it all off. The sole key female, Scarlett Johannson... just doesn't fit.
I'd give The Avengers a high mark for visual appeal and interesting main characters. However, the film also gets low marks for making poor use of the cast and an even poorer story line. Seems Jon Favreau and company cannot decide between making a Transformers film, sticking to what Marvel does best, or just spending his time setting up a series. The team, including Stan Lee, is stuck between playing tongue in cheek with it all and creating a film where the characters truly believe the world in which they live.
The potential is huge, and the market is waiting. There's always the next film and a theme park on its way.