Subtract 1 karma from yourself each time up-vote or down-vote is used; this effectively turns karma into "money" that can be spent on maintaining good discussion and high quality submissions.
This would make people think twice before down-voting. It would also reduce "knee-jerk" up-votes, as people would be more likely to reserve up-votes for things that truly deserve it.
It would also mean that long-time users will naturally do more of the moderation, as they have lots of points and may not be as afraid to use them.
This also makes karma a zero-sum game. This necessarily results in a distinct winners-and-losers scenario, which would be antagonistic to the goals of the site.
This would make people think twice before down-voting. It would also reduce "knee-jerk" up-votes, as people would be more likely to reserve up-votes for things that truly deserve it.
It would also mean that long-time users will naturally do more of the moderation, as they have lots of points and may not be as afraid to use them.