What annoys me slightly of downvotes on HN is that many users seem to simply downvote comments for the only reason that they don't agree with their content (as

What annoys me slightly of downvotes on HN is that many users seem to simply downvote comments for the only reason that they don't agree with their content (as opposed to judging them off-topic, spam, or poorly thought/ structured). One possible solution would be to force downvoters to leave a comment explaining the reason of the downvote. The comment could then in turn be up or downvoted, putting the karma of the downvoter at stake.