Well I think the point was that Banner could control the Hulk most of the time because he was "Always Angry" so he could keep from tripping because he always saw that edge before he transformed. He transformed on the Helicarrier because of two reasons, one was the spear was influencing all of them to be more hostile (that is why Bruce picked it up and didn't even realize it) and two the surprise of the attack.

During the big battle the Hulk had a focus for his rage so worked with those that were working against that focus. I think that he went out of his way to save Stark because out of all of the people there he was the most accepting of that side of Banner and those feelings filtered through the rage of the Hulk.

Anyway, in the comics Hulk is more intelligent and I hope they bring that into the movies too.