If I simply disagree with someone, I prefer to reply (or upvote someone else's reply) with an explanation of why I disagree. I try to only downvote posts with whom I disagree if I believe they are being mean.
As for my own posts, I do get frustrated when I am downvoted but nobody replies with an explanation for why they downvoted me. I leave comments on sites like HN because I want to evolve my own thoughts on the subject and help others evolve theirs. So to me a downvoted post with no replies is a missed opportunity to evolve my thoughts and better understand those who disagree with me. Unfortunately, it doesn't look like this site applies to HN comments.