I think this is an actively bad idea. If I had to reply to a comment, explaining why the comment is bad, then the most likely scenario is that the writer replies, arguing with me, and I reply, and now the thread is full of a bunch of bullshit that nobody wants to read. Multiply this by ten per post.
If the downvote reason was only visible to the writer, then I could go for that. But it's ridiculous to make a rule that drives people to have huge off-topic flamewars in the middle of a comments section.
Make the downvote reason only visible to the writer, and do not allow the writer to reply to the downvote reason - the system already heavily favors upvotes over downvotes, so that writers can just shrug off the downvotes.
The main thrust of my suggestion is to make downvoting even harder.
If the downvote reason was only visible to the writer, then I could go for that. But it's ridiculous to make a rule that drives people to have huge off-topic flamewars in the middle of a comments section.