The assumption that bad comments will get less votes is a canard. As the community grows, the same people adding comments also add their votes. If anything, they're more likely to vote since it's just a click, and so noob votes will drown old users even more. It's not just downvotes but any votes that have negative social consequences.
Update
[1] Exhibit A: http://news.ycombinator.com/item?id=1021157
>It's not just downvotes but any votes that have negative social consequences.
Upvotes also tend to select for popular modes of thinking. In the end, a system of only upvotes makes people want to only post things that will be perceived as popular by the community.
Positive reinforcement does have the advantage of encouraging more participation.
This is one of the rare cases where humor bought upvotes. I'd say for every 10 evenly mildly humorous comments, 9 of them are voted down.
But a) I see a couple of upvoted funnies everyday, and b) things are worse than they were a year ago. I'm joining in the fuss about it in hopes that we can stem the rot.
On digg the average quality of the posts dropped to near youtube levels, and on reddit the community fractured to extremes of opinion.
When I did get a response as to why, the self-justifications were fairly alarming.
Of course, if you notice it doesn't stop me, but that's because I'm a stubborn stick in the mud. I suspect that loads of people who are not as thick skinned as I am are afraid.
I mean if you weren't upvoted, I doubt that would discourage you from posting stuff.
Why? What have you to fear from being down voted?
But a down-vote has negative social consequences. It affects the types of comment that show up and shapes and refines the tone here so that eventually only the popular types show up.