Did you play it for more than five seconds?
There are no classes for humans, only zombies. The first zombie is a carrier who has more hp, is faster and can infect (they turn into a zombie after a certain amount of time) You can barricade pretty easily, as long as you move the sofas/chairs/tables with rmc when your weapon is holstered.
How is it flawed? It's not that off-balanced as long as you stay in a team. Just pick up a decent gun and stick with a couple of guys at the top of the leader-board and you'll be sailing.
If you're a zombie use stealth or you're dead, cause just running in and trying to kill everyone just uses up lives. Hide behind something, sprint to them and you'll find they fall quicker.