Brainbread is fun for a while, but zombie bots aren't very fun for me. I find it more fun to go against an opponent with instincts.
In Brainbread, find an ammo chest and camp it. In ZP: Source, find whatever the hell you can, and conserve it. I love how detailed the maps are. I mainly love setting up barricades. Makes it feel more like survival, and not just running around killing as many as you can. You're actually trying to survive the zombie apocalypse.
I like the skill balance as well. A good zombie (most often a carrier) can take down 1-2 guys out of a group of 5 before getting gunned down. But that's if he's good. If he's not, he'll blow it and get killed instantly.
Humans have to watch what they carry. Too much weight slows you down and that can be really bad in close quarters against a horde of zombies. It's just fun having to think in survival mode. What you need, as opposed to what you want. That's how the game is built, and it's built damn fantastically. With a good group of players, it's easy to come up with fun strategies for surviving while simultaneously beating back the zombies until you win.
Overall, those reasons are what make the game completely ****ing awesome to me. You HAVE to work as a team, or you're guaranteed to die.
Edit: Oh, and I assume they hit you because it'd be a pain in the ass to make a biting animation that didn't look like ass. But who cares? It's fun anyway, and you're the first person I've heard complain about it.