Zombies or the "Infected" need to be tougher, stronger, meaner, but definitely slower (but no slower than a player), and have poorer vision then they currently have. Oh and there need to be more of them and they need a group aggro system where if you aggro one zombie every zombie within 50 meters of it will also be aggroed, and this needs to have a cascading effect until the entire town/city is chasing your ass. Though conversely, they should focus on the wounded, ignoring perfectly healthy players to hone in on the cripple and strip him down to the bone, while also being stupider being that if they lose contact for more then 10 seconds or until it takes them to reach last known location, they should immediately give up the chase and disperse back into the city. In this way zombies will be more of a threat and consideration for people firing off loud weapons in city limits, or anywhere near a town without proper back up.