I havent played much of it myself, but, they just make a b-line for you and have a pathetic attack which is rather boring. Maybe different classes of zombies could spice up combat a bit, such as extremely fast zombies with little health or giant ones with like 6 headshots needed for a kill. These could maybe vary using colour to identify them too. At the moment it's too simple to be fun for a long time, but, then again, it is a free game.