Zombies are great. I think Tinker said it before - they're just an awesome game opponent. Everything can be explained when you're shooting zombies.
"The animation on that zombie looks all crappy"
"Of course it does. It's a zombie. They walk like cripples"
"It's just running straight at me ... oh and its fallen into that river"
"Of course it has. Its a zombie. It's stupid"
"Look, that zombie is stuck behind a tiny little piece of wood on the floor and is running on the spot"
"Of course it is. It's a zombie. It doesnt have good hand eye coordination nor advanced obstacle negotiation theory"
"I shot it 100 times with a 70mm howitzer and its still alive"
"Of course it is, it's a zombie. They're never die ... ever, EVER!"
"All the zombies look exactly the same! Whats up with that?"
"Of course they do! Have you ever seen a zombie? No. They all look like that. Now shut up."
etc etc.
See? Zombies are the ultimate enemy. Nobody can diss the zombie.