I downvote comments that I feel are not constructive to the conversation. This includes things like unnecessary aggression / attacking, spreading misinformation, using offensive slurs or language, and trolling. I don't downvote based on differences of opinion - in fact I upvote people who disagree with me if they've engaged in a respectful and genuine way. I also upvote comments that I think are well written and insightful. Sometimes I upvote comments that have been downvoted and I can't see a valid reason as to why.