I really enjoy playing Zombie Massacre, but one thing has always bogged down the experience for me.
Its that someone always has to be Doctor, and if no one is Doctor, the team dies much faster, and on harder difficulties its impossible to play without someone playing as the Doctor class.
If everyone chooses to be DPS, which they usually do, your health is static. There is no health pickups, no recharging health, If you get hurt, its pretty much over for your character, because you might encounter an enemy that could one hit kill you out of nowhere like I often have.
One issue I have with the Doctor class is that there really is no incentive to play as it. You get less score since all your teammates have abilities that can kill more zombies, If healing your teammates gave you score, people would probably want to play as Doctor more, but this issue would still effect the game mode.