1. Outright abusive language, personal attack on someone, sarcasm or obvious self promotion.
2. Irrelevant to the actual discussion even if it is otherwise well written. Some examples: humor/joke without any other valuable discussion, talking about a totally different topic.
I never downvote even if I disagree with someone as long as they are not in one of the 2 categories above.I also don't downvote if someone makes an honest mistake or is wrong about something.