One (major?) change I'd like to see is requiring the user to post a reason when they downvote something. This would not appear as a reply to the post itself, but rather be available for viewers in a pop-up box when they mouse-over the post.

I think this would really solve the problem with rampant downvoting of things people disagree with (as opposed to downvoting low quality content, which is what it's supposed to be for).