I know it sounds strange, but my fear is only triggered by a few specific sets of characteristics that if they are not in the correct alignment I am not as afraid. You see my fiance pointed this out to me just a few weeks ago, I am not afraid of the rabid humans from "I am Legend", when by all obvious accounts I should be. In fact, the level of virulence is so very high in that film that it is almost unbelievable. The people mutated by the virus still ate everyone else, they still destroyed humanity in almost it's entirety. I am not sure what it is about this that helps me to watch the film - though I admit I am still nervous about the virulence of the disease. In fact, I am more worried about the virus than I am the rage-filled mutants eating and killing everyone.
No, my fear really is about a very specific set of things that must be in place for zombies to terrorize me. Virus - yes. Eating people - yes. Stumbling rotting flesh bent on mindless consumption that is society - yes.
I am not sure I will ever figure out what specifically qualifies a specific thing to become terrifying, but I am sure it is in there somewhere. I guess as the saying goes - the devil is in the details.