I don't mind aiming to the head, in fact, I like some difficulty for killing zombies, but they do too much damage and they can destroy everything you have with a few hits.
In theory, zombies are strong when there are a lot of them, and I think that one zombie shouldn't be a problem if you have good weapons and good aim, but that's not the case.
In my opinion, speed and damage (specially the damage they do in the gear) must be decreased.