It's a little different on HN due to how the comments are hidden quite quickly when downvoted. I frequently upvote people who reply to me but I disagree with, simply because it would ruin the comment chain and hey, they might just be right anyway ;)

I reserve downvotes for rudeness and demonstrably wrong/misleading comments.

I can understand a downvote with a solid reply, but downvote just for disagreeing doesn't add to the discussion, its more of a censorship which should only be used for offensive/misleading comments IMHO.

Personally, I use upvotes to reward new knowledge or an interesting argument in a comment, I don't upvote every comment I agree with, and downvote only if the comment is offensive.

So then the end result is that opinions/comments that a lot of people disagree with will get punished? Seems counter-intuitive if you're trying to have a reasonable argument/discussion/debate about something. It's one thing to express disagreement, but it's completely different to silence + punish someone because you disagree with them.