Related to: Well-Kept Gardens Die By Pacifism.
I wrote a script for the greasemonkey extension for Firefox, implementing less painful downvoting. It inserts a button "Vote boo" in addition to "Vote up" and "Vote down" for each comment. Pressing this button has 30% chance of resulting in downvoting the comment, which is on average equivalent to taking 0.3 points of rating. If pressing the button once has no effect, don't press it twice: the action is already performed, resulting in one of the two possible outcomes.
The idea is to lower the level of punishment from downvoting, thus making it easier to downvote average mediocre comments, not just remarkably bad ones. Systematically downvoting mediocre comments should make their expected rating negative, creating an incentive to focus more on making high-quality comments, and punishing systematic mediocrity. At the same time, low penalty for average comments (implemented through stochastic downvoting) allows to still make them freely, which is essential for supporting a discussion. Contributors may see positive rating of good comments as currency for which they can buy a limited number of discussion-supporting average comments.
The "Vote boo" option is not to be taken lightly, one should understand a comment before declaring it mediocre. If you are not sure, don't vote. If comment is visibly a simple passing remark, or of mediocre quality, press the button.