Posting an explanation for a down-vote is very rare. It should be a requirement for the privilege of down-voting on HN. It is also, in my opinion, good manners.
I'd be fine with requiring an explanation that was visible only to the down-voted poster. I actually wish that more people would down-vote more freely, in the hope that low-quality comments would be more discouraged than they currently are.
HN could easily make down-voting comments a requirement and then add a simply feature by which down-vote comments are folded below the original post and not visible. You could then have a "view down-vote comments" link that would reveal that thread. In other words, keep them out of the main conversation so that they don't clutter things yet make them available for review by interested parties.
I see that, but am prioritizing the good of the group over the re-education of the individual. I think the majority of new posters will catch on to the norms of the site quickly, and those that don't will likely move on if they don't get responses.
HN could easily make down-voting comments a requirement and then add a simply feature by which down-vote comments are folded below the original post and not visible.
Fine idea. I posted it as a feature request here: http://news.ycombinator.com/item?id=4209304