I actually have to agree with sexpy, he does bring up a good point. And since there are certain members of this forum who are close to one another and tend to pick favorites/not-favorites, it'd just be the same people positive/negative repping each other and circlejerking, though there are limits of course. Not like it matters how much rep you have, but still....
Reputation systems, I find, are more fitting on image boards. I used to be a part of a site of a certain nature in which people would contribute images, and in turn they would receive reputation for their contributions. It sure as hell is an ego-booster, but on a site like this I don't think it would be necessary.
Not like the topic at hand is arguing whether or not to have a rep system, but that's just my 2 cents I guess.