The performances are just fine, as believable as they need to be, but not falling over into complete parody or pretense. The advantage for actors in a movie like this is that nobody really knows what a superhero acts like, though there are hundreds upon hundreds of unbelievable and ham-fisted examples.
When creating any number of supermen, a little goes a long way. Chris Evans (Captain America) gives us the earnest wartime hero that he should be. He acts like he stepped out of a John Wayne WW2 movie. Scarlett Johansson (Black Widow) mostly pulls off the vampy spy hero, manages to keep her sex appeal in service of the film and character. Though, sometimes her coldness plays very close to a vapidity that does not do her justice. Mark Ruffalo (The Hulk) is the most exciting discovery in the cast. The discovery being the human being that he found within the role of the green monster. Sorry, Bill Bixby, but most Hulk performances serve as filler between the smashing, and Ruffalo makes the time before the smashing more interesting than the tantrums themselves. Likewise, Chris Hemsworth (Thor) gets rid of a good amount of the camp that infected Thor and gives us a Norse god who is less Point Break.
Jeremy Renner (Hawkeye) is slightly more interesting when he’s bad, as he’s got a bit of boy wonder to him otherwise. He doesn’t quite sell as a good assassin as well as he does an evil one. Tom Hiddleston (Loki) gets to the heart of a god of mischief, but he is sometimes a little too clever to be threatening. He could’ve studied some Gary Oldman or classic Jack Nicholson to bring some more menace to his charms. Robert Downey, Jr. demonstrates his exceptional comedy chops and ability to deliver one-liners, which you might not get until after the scene, has ended. This wit makes Tony Stark’s genius all the more believable. Essentially supporting actors Samuel Jackson, Stellan Skarsgard, Clark Gregg and Cobie Smulders play along well and never let the audience think they don’t believe in the story themselves.