One of the things I like about Slashdot’s system is that it requires a reason for a downvote. Of course that doesn’t prevent people from downvoting disingenuously, but it nudges users away from downvoting just because they disagree.

I think for most social systems, the UI I’d use is a report or flag button that pops up a second step with a list of reasons, and like Slashdot, show the most selected reason next to low-ranked posts.