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.
I'm still making comments. And I value some strategies over others to get karma - I don't want it to go up for sheer volume and dumb luck. I'd rather make comments that come to mind as good things to say, not just type the unfiltered contents of my brain. (The unfiltered contents of my brain contain a lot more fiction references, inside jokes that are known only to my close personal friends, and musings about muffins and tofu and broccoli than anyone wants to read.) And I'd rather the good ones get voted up and the bad ones get informative replies - not freely dispensed downvotes - so I can adjust my behavior with more information than a brute negative number.
Also, as a counterexample, I took a karma hit for a number of comments on another thread - I remarked on it and they've all been compensated for, but if I hadn't said anything I imagine they'd have sat there indefinitely, because apparently the new "downvote freely" ethos hasn't made me paranoid enough.
For comparison, I prefer to comment the same way I would if nobody was voting on them. If the people here like my comments, then they'll float to the top. If my comments routinely got downvoted to oblivion, I'd probably post elsewhere instead.
In other words, I'm a big fan of the marketplace of ideas, but I don't have one in my own head.