I'm one of the few who likes fast zombies, because I find them even more threatening and frightening (they were the major reason the Dawn of the Dead remake disturbed me so much...the utter hopelessness of the situation really got to me). But I agree that they don't belong in this movie. Since WWZ was based off the Zombie Survival Guide, and that was focused only on how to deal with slow zombies, making them into fast zombies is destroying an integral part of the source material.

Then again, so is making it into a straightforward action movie instead of a mockumentary. I was always hoping they would go the mockumentary route, but I understand that sort of film has limited appeal, and they probably felt like an action movie was a safer bet (and there are some cool scenes in the book that would look great in an action movie). I suppose if they're going to change their approach to the source material so radically, changing the type of zombie is only a natural consequence of that.

That said, I still remain optimistic. It looks like it could still turn out to be a kick-ass zombie movie in its own right, even if its connection to the source material is tenuous