If you want to tell people how to use a self-moderating site, you are likely setting yourself up for failure. Either hire moderators, go to Slashdot's system of rewarding certain behaviors with moderator points, or live with the digg/reddit consequences.

Also, controlling how people downvote things is going to be really tough. Many people think comments that they disagree with do detract from conversation. I think one obvious rule should be you can't downvote comments on stories you comment on yourself.