Re: The Avengers (2012)
^^^ When he was acting all crazy on the SHIELD heli-carrier, it was because Loki had been subliminally influencing him since he first stepped aboard. After, when Banner joins the group for the final battle, it's without Loki's influence so he's able to -- to borrow a line from Hulk '08 -- actually "aim" the Hulk at the baddies rather than just unleash him to destroy whatever.
Azzie -- responding to your last post, the villain in the after-credits is a MAJOR fan-favorite villain in the comics and has had run-ins with pretty much everybody, from the cosmic powerhouses like Silver Surfer to the more street-level heroes like Spider-Man, thus all the buzz his appearance created among fanboys. Interestingly enough, though, it appears that particular villain is actually going to be appearing in a new Marvel movie (called Guardians of the Galaxy) rather than Avengers 2.