Why not give the Hulk flight capabilities?  

@cosmicx: His leaps come pretty close to flying as it is. He doesn't really need flight in most situations.

Aside from the aforementioned 'He jumps high' argument, the fact that he CAN'T fly is what allows most other Marvel superheroes to sleep soundly at night: The Hulk can jump, yes, but if he could fly? There'd be no escaping him. Heroes with flight capability know that if they see the Hulk, they can get airborne and high enough that he can't reach them. They have a way to escape him if necessary. That's what they hold onto to keep themselves from defecating at the sight of the Jade Giant.

But if that advantage was taken away? If they suddenly had to deal with a flying Hulk, whose only getting stronger by the second? Well, let's just say there will be a lot of heroes changing their costume color to a nice shade of brown.

@JonSmith: What they want to fly?

If Hulk without any problems from the moon to the planet jumps.

(In seconds)