The primary fear the zombie movies play upon is being eaten alive. To that extent, fast zombies better emulate wild animals, and so amplify the predator/prey instinct in the back of the mind.
I'm glad that a majority of you find slow zombies scary and fast zombies silly. While you're busy mocking how silly it is for a reanimated corpse to have normal reflexes after rigor mortis would be dissipating, I can run away and make sure you're the closest source of food.
I have no problem with the idea that a recently deceased/reanimated zombie could be fast moving. A recently deceased corpse should be more mobile/agile than a long desiccated one.
WWZ is interesting in that it suggests, at least in the books, that Zack does not decay. Whatever it is that reanimates the corpse makes the body less desireable as food for the bacteria that would otherwise consume us after we are dead. Additionally, the "virus" converts most of the body's fluid into some sort of black gel/goo that helps it retain some moisture and keeps things lubricated. Without decay, the only thing that would prevent a zombie from running is a lack of sufficient energy...but we hand wave over that given that they owuld need the same energy in order to move at all.