I don't think downvotes should be used for disagreeing.
Right; that was intended to mean that people sometimes use them for disagreement, despite that they probably shouldn't. Any thoughts on a better wording?
Upvotes should reward civil comments that feed the conversation constructively.
Aggressivity, and empty posts ("cheerleaders", etc.) should be downvoted.
Flags are for grossly offensive or illegal content.
That was the original intent IIRC: encouraging civil conversation. Tying votes with agreement leads to groupthink. We should encourage diversity unless we want to become a close-minded community, reinforcing it's own biases.