On moderation.
This reddit.com thread has enlightened me to the simple effectiveness of the Slashdot comment moderation system. For all its troubles, it’s still the best out there, and here’s why: Moderations are all positive, except for inappropriate or abuse flags, like troll/spam/offtopic/redundant. This tends to discourage downmodding comments which are on-topic but with which you disagree. Contrast Digg and Reddit: people seem to use the -1 button (down-arrow in Reddit, thumbs-down in Digg) to bury comments they disagree with.
The way I see it, if you can cause others’ arguments to disappear from conversations just by the vehemence of your disapproval, you ruin any chance at meaningful discourse. Because Digg and Reddit allow you to mod down for any reason, users feel freer to mod down for the wrong reasons.
(I’m still not entirely sure where I fall on the whole “everyone gets mod powers” vs. “only the karmic get mod points, and then only sparingly” debate. Thoughts?)