The problem is that downvoted comments are less prominent- they're all the way at the bottom of the page (and greyed out). Thus if people start downvoting thing

The problem is that downvoted comments are less prominent- they're all the way at the bottom of the page (and greyed out). Thus if people start downvoting things they just disagree with, things that contribute to discussions won't actually be seen, reducing the quality of discussions. Karma isn't the problem.