The problem with slow zombies is that they're only a real threat in large numbers. If you're faced with only one or two, there's no horror or challenge, because all you need is a baseball bat, or even to just walk away at a moderate pace. It's only when their numbers swell to the hundreds or thousands that they become a real threat, because then they can surround you, and their combined weight and force can overwhelm any barricades you put up.
Fast zombies, by contrast, are a threat even singly, even to groups of people. A single fast zombie can infect a small group of people all on its own, unless they're able to coordinate quickly and efficiently enough. Even then, it's likely that at least one or two will end up going down.
That being said, while I'm looking forward to World War Z, I really wish they had kept the format of the book, with slow zombies and telling different stories of the war in a series of flashbacks, as well as how people adapt to the new world where zombies, though greatly reduced in number, are still an ongoing threat.