We don't need this feature. It's just going to encourage people to downvote posts if they think it has too many upvotes or vice versa. "I wasn't going to downvote that, but it had 6 upvotes, it's a good post but not THAT good, so I took it down a notch." Downvotes don't add anything to the discussion.
At least if you write a "mean" reply to explain why someone is wrong, you put yourself at risk of criticism. Where is the risk to downvoting? It makes it too easy to be negative.