Lag
Far far too much lag, and the problems are almost limitless. The actionscript needs some serious tinkering to make it way shorter and less glitchy. Oh no, I pick up a sniper rifle, now I can't use the rockets I had. Kabam! Oh dear, my shot went straight through his neck and out the other side and he hasn't noticed. Bang! Now I've been shot by someone apparently at the other side of the arena. Boom! As I respawn someone blew the two of us up with a grenade launcher, not giving a point to both of us but 2 to him instead. Now the kills are racking up, the lag is terrible, I can't see anyone but they can see me, now I'm dead again while my team mate is running into a wall. Is this really fun?
Also, make a motive for completing the single player because it seems better to just make a deathmatch and be done with it; it's more fun anyway.