Kevin Feige, president of Marvel Studios and producer of The Avengers (2012) in an interview with "Hero Complex" said the following:
It will be as different as "The Incredible Hulk" was from Ang Lee's "Hulk" in terms of its look and design, but it is Hulk.
As you can already see in that concept painting, it is — more than any Hulk that's ever been done in live action — a Hulk that let's you see the actor in there. You will be able to see Ruffalo in there. That was a big revelation for us. It's just a concept painting, but in that you can see the [influence of Marvel Comics pioneer and original Hulk artist Jack] Kirby, as people have already pointed out, but also, and equally important in this case, Ruffalo's eyes and his cheek structure. It is him.
So how different will the Avenger's Hulk be? He's implying that other Hulk's haven't really looked like the actor. Is that true?
Hulk (2003) with Eric Bana
Artist: James Carson
This looks quite a bit like Eric Bana. They widened his jaw and shortened his nose a bit. The eyebrow ridge is more pronounced and his jaw is much larger than his real one. The entire face looks like it was blown up like a balloon.
Its hard to tell from this angle, but at first glance it looks like they took the same liberties as they did with the other films. Eyebrow ridge is larger. Nose is shorter. When you look closer though, you do see some changes.
Ruffalo's face is thicker, so it's easier to bulk it out. His jawline, while larger, isn't that much larger. The same with most of his features. He still looks like the traditional Hulk, but his face doesn't change as much.
It's possible that they chose him specifically because his face lends itself to the distortion. I've never done casting or concept design though. So, who knows?
In the end, it is different, but mainly because the actors face is Hulk-like.
Update: In the book Art of Marvel's The Avengers there was a quote from Ryan Meinderding that supports my viewpoint.
Joss really pushed us to take some of the cuses from those early Hulks and combine them with Mark Ruffalo's features. At first, we couldn't see how that would work - but he was dead-on in that direction. Mark's face really lends itself to hulking out, and we affectionally called the design "Hulkalo."
What do you think? Is it really a "different" Hulk?