Two things. One, the zombies look too made up...a little too perfect. You can tell that they're just people wearing a ton of makeup. Two, computerized zombie kills. Did we really need a CG decapitation? That's just lazy film making. Digital blood splatter? Come on. It just makes the whole thing so cheesy.
If you want to see perfect zombie special effects, check out the Night of the Living Dead remake from 1990.