If there's anyone out there who doesn't think that 'The Incredible Hulk' was the weakest of the "Marvel Universe" cycle of movies, I haven't heard from them. The consensus seems to be that while it wasn't a failure like the previous, disastrous Ang Lee 'Hulk', it's certainly nothing like the show-stopping success enjoyed by two Iron Man movies, Thor, and Captain America...and that 'The Avengers' showed a possible blueprint for how to make a Hulk film work. But in an ensemble film where just about everything worked, what is it specifically that made the Hulk click in a way that he hadn't in two previous films? What did Joss Whedon do right that Ang Lee and Louis Leterrier did wrong?
The answer is simple. The Hulk spoke.
Technically, he sorta spoke in all three of his film appearances. In Ang Lee's 'Hulk', during one of the film's weird, incoherent dream sequence-y things, he said, "Puny humans," while in 'The Incredible Hulk', he shouted "Hulk smash!" at the film's climax. That's roughly the same amount of dialogue he had in 'Avengers'. But in 'Avengers', he had one of the film's most memorable, laugh-out-loud, stand-up-and-cheer lines: "Puny god," spoken after treating Loki like a particularly maladjusted toddler treats a Betsy-Wetsy doll, showed the Hulk's actual personality in a way we never saw in the previous films. It showed that the Hulk isn't just a mindless force of nature, he's a person. He has his own way of looking at the world. And while he's frequently terrifying, as a being that powerful can be, he's actually just a big dumb goofy kid inside sometimes.
The comics have been embarrassed about the Hulk's goofiness for ages. I'm not sure exactly when it happened, although Peter David's decade-long run is certainly a place to start. He famously only used the Hulk's stylized "Hulk smash puny humans!" style of dialogue that Roy Thomas invented in a very limited capacity, and usually for comic effect to contrast it with the erudite and sarcastic Hulk he was more interested in writing. And while that worked for Peter David, it's worth noting that many of his "dumb Hulk" lines were also drop-dead funny, including the awesome comment made by the green Hulk in Banner's body: "Hulk's butt hurts!" Let's face it--Hulkspeak is freaking hilarious, and it's not by accident. It's meant to sound silly. It's meant to make the Hulk a little bit sympathetic, which is needed given that he's a brute and a monster.
The films we've gotten have refused to do that. They've been obviously trying to avoid embracing the silliness inherent in the Hulk's character, and it's hard to make a film work when your director refuses to stand behind its own central idea. (Notably, Ang Lee never lets the character be called "Hulk" onscreen. That takes being ashamed of your comic-book roots to new heights.) What's needed is a film that lets the Hulk be silly and sympathetic, while never letting us forget that he's dangerous and powerful. A film where the Hulk punches the giant flying dragon-worm...and then shoulder-punches Thor clean off-camera because he's in a bad mood. It needs to be funny as well as serious, something that the previous two attempts didn't even go for.
In short, the Hulk needs to talk. And he needs to talk goofy. If you can't get behind that, scrap your draft script and go back to the computer, Mister Screenwriter.