Downvotes can be accidental; incorrect downvotes are normally corrected pretty quickly; commenting on your downvotes is risky because that can attract more downvotes; downvoting for simple disagreement is acceptable on HN.
Downvoting for disagreement is allowed, but I still think it's a dumb policy. I upvote people I disagree with all the time for moving discussion along with pointed questions, so it's saddening to see people downvoted or be downvoted myself merely for holding an opinion contrary to the hivemind. Not because I give a flying fuck about internet points, but because it reflects poorly on the community and its capacity for debate. But then again, I think upvote systems are dumb to begin with.