We had that on my old board. Actually, it was a user's rating.

I was against it at first, as the conservative mod that I was, because I thought it might have been used to troll. Fortunately, I lost to the majority and it turned out to be kind of great.

You could give "+" points to a user according to what he/she posted.

After a while, everybody could see if you were behaving like an asshole and it helped a lot. People were suddenly really putting an effort into their posts.

I'm not sure this would work here though...