If negative or down votes were allowed, I wonder how many would actually use the feature constructively? Especially if it reduced the payment of the author. Humans being human, it's easy to imagine a person getting angry at how his point of view was being received and downvoting those who disagreed with him. I also think very few people would downvote anyone with significant SP out of fear of retaliation. I guess the real question is, what appropriate criteria should be used for negative voting? I agree that it's needed in order to have a balanced feedback system. But how to best implement it so that it's constructive and not abused?