People who go back and downvote every post or comment a Less Wrong user has ever made, please, stop doing that. It's a clever way to pull information cascades in your direction but it is clearly an abuse of the content filtering system. It's also highly dishonorable. If you truly must use such tactics then downvoting a few of your enemy's top level posts is much less evil; your enemy loses the karma and takes the hint without your severely biasing the public perception of Less Wrong's discourse.
(I just lost over 200 karma in a few minutes and that'll probably continue for awhile. This happens to me every few weeks. Edit: I mean it's been happening every few weeks for a few months for a total of only three or four. Between 400 and 700 karma lost total I think? I don't mean to overstate the problem.)
I don't really get why people downvote at all. I don't do so unless I feel very strongly about something - a comment explaining my disagreement or disapproval is more productive, doesn't make the downvotee feel bad, and I frequently get karma because people will upvote my comment. Win-win-win.
Asymmetric voting reduces the informativeness of the karma ratings. See also EY's point here: that if views are distributed evenly, but people speak up only when they're on one of the sides, then you get the wrong impression about the consensus.