What if there were anti-lurve?

I am NOT suggesting that Fluther.com implement this, but does anyone have any experience with (or well-considered thoughts on) sites like this that allow both a ‘thumbs up’ and a ‘thumbs down’ vote on questions and answers?

Does it affect the number of questions and answers?
Does it effect the quality of questions and answers?
Does it effect spontaneity? humor? smart-aleckness?
Does it effect the fun of the site?
Does it effect the membership of the site?