What if people who up-vote nasty comments don't really do it consistently, they just vote a lot? Maybe votes need to be precious, so people use them more carefully. Slashdot addressed the problem by limiting the mod points available and introducing meta-moderation. Meta-moderation won't work in an up/down voting system like this, but limiting votes, for example to a certain number per day, is a powerful option. You could even have tiers, where people with more karma would get more votes per day. Maybe every 100 karma points gets you an extra vote per day.