For example, I'll sometimes see a massively downvoted comment that's constructive and beautifully written, but it carries an opinion that is not currently popular for whatever reason.
I believe that downvoting someone just because you disagree with them is terrible for a discussion network like HN. Moreover, I would say that I learned the most from the comments that I disagreed with - on all levels, politically, technologically, personally.
I will downvote a comment if it's not constructive, only written to be provocative, etc. It doesn't matter if I agree with the author's opinion or not. If someone puts in the effort to explain their opinion politely and constructively, my thinking is that it's positive for all the people in the community.
So, what kind of a downvoter are you? What are some of your reasons for downvoting a comment?