As a writer (hack, if you will), I know suspension of disbelief is vital if I'm going to take the reader on any kind of a journey. When I wear my "reader" costume, I've been known to put a book/story/what-have-you down when the writer violated that sacred trust.
Okay, so here are the problems my brain cooks up at two in the morning (insomnia, anyone?):
1. Why don't zombies eat each other? They obviously don't care that much about sanitation (just look at them) and are pretty indiscriminate about what they put in their mouths. I'm sure somebody has cooked up the "virus doesn't taste good" or "don't eat their own kind" argument, but that's just lame. In the real world, the whole zombie problem would probably be over in a few hours after the outbreak, just after they devour each other. The National Guard can wait around and pick off remainders.
2. Vampires. The whole "handsome, super strong, nearly invincible" thing. These are the living dead, folks. Only a few notches above aforementioned zombies on the food chain. How/why they became the fodder for paranormal romance (shudders) instead of the thinly-veiled metaphor for venereal disease they once were, I will never know. Look, at a minimum, their breath has to reek. And super-strength? Have you tried to subsist on a diet of blood? I couldn't even play a decent game of tennis when I...er, never mind.
I could go on, but I think I may have revealed a little too much about how my brain works at two AM. I love zombies...I don't want them to eat each other because it's much more fun when they try to eat us. Vampires? Meh. I prefer the nasty, monstrous type. Good old Vlad the Impaler type. You can take Fabio the Bloodsucker and launch his sorry butt into space with all the other debris.
Update: Catherine J. Gardner is our guessing-game victor! Huzzah!