I don’t even think it would be good to require a private explanation when downvoting or flagging. In my experience with other services with user-generated content, negative feedback signals for community driven moderation are very valuable and most users never give them. You want the process to be as streamlined as possible. You can give more weight to feedback from more trusted users, which HN does in a transparent way by gating the flag and downvote options to accounts with more reputation.
To be clear, I think the comment here is a good contribution. There’s a lot of passion about this topic and the system seems to break down somewhat.