I've been playing this game a lot. It's pretty addicting, like Counter Strike used to be back in the day. (Before I played every map to death...) I think it's the same thing that made CS so good: If you die you actually have to wait until the next round to play again. It makes you want to stay alive, instead of rushing to your doom over and over like a deathmatch, instant-respawn game. Also, the buying weapons and powers adds a nice layer.
Now, they need to add more guns, more maps, and at least throw an Ork player model in.