Thor and Hulk are fighters, and Peter and Hank are full on nerds. Tony is a nerd but he's also a business man, so he knows how to budget and deal with the politicians (pre-Extremis?). Steve is a solider, he's been in a chain of command structure so leading is natural to him, as is following when the other person has the better idea. I love how when the group splits up, Iron Man and Cap always go together, and cooperate equally so well.

From the few scenes I've seen of Wasp, she takes a lot of initiative. In Marvel Adventures, Storm is cast as the seasoned leader. Steve directs them in the field, but Storm directs them off-field and point out areas of improvement in a constructive manner. Hmmm, in fact, Storm seem to lead more than Steve and Tony, who somehow seem younger. This might account for the fluffy atmosphere, things are always fluffier when there is a grown up doing the spotting.