Spoilers. So don't read if you haven't seen the movie.
Okay. When Banner first transformed into the Hulk in the film he was in the presence of Black Widow. After transforming he instantly attacks Black Widow and then Thor. This tells us that Banner hasn't exactly began to control the Hulk and will attack anything in site when in this site. Bruce knew who they all were, become friends with them and whatnot.. But when he turns to Hulk he berserks on everything and everyone on site.
Later in the film in the ending. Bruce appears during the battle via scooter and instantly joins the fight. He begins helping out the other Avengers and looks to be a great teammate, not attacking everyone in sight (other than Chitauri and Loki), even hanging out with his new teammates once the battle ended in the Hulk 'form'. Did I miss something or did Bruce learn to control Hulk within a few hours?