Another solution is to change the karma system to remove the psychological obstacles that may keep people from downvoting. It feels a little mean to directly cause a comment to be filtered, even when it would probably improve the mean quality of discourse. It may be a little easier to express your opinion that a comment is not constructive, and have a less direct mechanism responsible for converting a consensus into moderation / karma penalty.

I do this by keeping kibitzing turned off most of the time, and always showing all comments regardless of karma. This won't work for everyone, but it works for me: I think my upvotes and downvotes are less biased this way.