I try to do the opposite (although I fail at times to) - downvoting for simply disagreement amounts to squelching dissent on a whim. If you downvote, at least a response as to why would be good. Otherwise, it amounts to communal censorship.
I'm not a fan of voting systems that let you downvote in general though.
Downvotes are sometimes useful for getting users to fight spam. One interesting solution I encountered on lobste.rs is to ask for a reason (out of a small list) before allowing me to downvote. It sure stopped me from censoring stuff that I find annoying.
I'm not a fan of voting systems that let you downvote in general though.