At 600+ karma, you get infinite bonus mushrooms and infinite ammo. You have to beat the game twice on impossible mode in the military suit and max out all your guns, though, even the stupid one that takes forever to fire and then the zombie eats your head because you were sitting there trying to warm it up instead of shooting zombies, which would have been the logical thing to do.

At 1000+ karma, you take off your helmet and guess what you're a chick. If you win three times in a row on impossible mode with perfect karma without getting shot once, using nothing but the annoying gun that takes forever, you take off your armor and guess what you're a hot chick. Then you can play the game again wearing a bikini.

At 2000+ karma, Paul Graham comes to your house, tells you the meaning of life, and makes you a sandwich. It is the best sandwich known to man. Only rich people can make this sandwich.