I've been on some forums which use the karma/reputation system, and none of them seemed to have gotten any benefit from it. Usually it devolves into yet another way to flame someone. Posters who don't like each other are constantly in neg-rep battles, and the posters with already huge egos have yet another statistic with which to inflate it further. Nothing that can be done with these systems can't currently already be done. If someone makes a thoughtful post, reply saying so. If someone does something stupid, report it or ignore it.
Besides, there's already too much clutter on this forum because it allows animated avatars. I want my forums simple. Plain backgrounds. Consistent font, size and color in posts. Small, motionless avatars or none at all. If I wanted bright colors flying around the screen enough to make me dizzy, I'd turn on the TV for a change.