Person feel obliged to register their opinion on some question. And upvoting appears the only acceptable way to do this (since "me too"-kind responses get downvoted quickly). So I think the ways to increase voting quality are: 1) allow to register user's agreement/disagreement with the post separately of the post itself (like another voting button), or 2) say in rules that you can't display your agreement or disagreement by upvoting. I think 2 is the best, since users will be forced to make sufficient contribution when they agree or disagree.