I must confess that prior to reaching a 50 limit, around about my 100th post, I felt more motivated to contribute. Since then, less so, but that isn't a bad thing because there is seldom a lack of good contributions on
There's an interesting edge effect pertaining to the 50 cap. If you have 50, then no matter how many people mod your articles up, the points have no effect, but if one person mods an article down, then (at least until next time someone mods you up) your karma score goes down.
There's an interesting discussion, in Stephen Jay Gould's Full House, of how a similar edge effect in nature can give the appearance of a pattern of a directional evolutionary force towards increasing complexity.