I am aware that you are planning to make some changes to the zombies such as increasing the numbers however one thing that I feel needs to be done is a nerfing of the zombies health it breaks immersion and is extremely annoying that it can take up to 7 hits from a fire axe to kill a zombie in my opinion if possible zombies similar to the game left 4 dead would be and improvement (much more however die with 1 or 2 melee attacks) the zombie threat seems artificial because of their health. The game touts for realism however I'm pretty sure if somebodies head was split in two with a fire axe they wouldn't be getting back up zombie or not.
in Retrospect this is most likely an issue with server lag and hit detection and because of this should be closed.
One point that has been made however is that the lower end weapons eg fists screwdriver take far to long to kill a zombie in comparison to the more beefy weapons such as the fire axe.