For me, the vast majority of down votes come from me being wrong (that is, I post something that I think is correct; I get down votes; I do more research and discover that I am, indeed, incorrect). In fact, this is such a good flag for my writing that when I get a down vote I immediately assume I must have made a mistake somewhere and start looking for it.
The next most common source of down votes (for me) is being argumentative. If I get a down vote, but decide that I am correct, I often discover that the tone of my writing is derisive. In all honesty, I never intend to have this tone of writing, but I am human and sometimes (either through lack of writing skill, or by unconsciously being an ass) it happens. My guess is that the person I'm responding too gets pissed off and down votes me. This has led me to have a kind of rule to try very hard never to piss off the parent poster, even if I think they are wrong. If I can't think of a way to comment without pissing off the parent, I try not to post. As a huge bonus, this avoids me getting dragged into >90% of trolling efforts (a statistic that I made up).
The next most common cause of down votes for me is lack of coherency in the comment. Sometimes people simply don't understand what I'm saying. It is important to realise that as a writer, you can't assume the readers understand what you are thinking, so this is completely down to you. Trying to put yourself into the role of a random reader and figuring out what they might think when they read your post is hard. However, this is what being a good writer is about. Again, if I write a post and upon reviewing it, I don't think most people will understand, I don't submit it.
Related to the last point, there are lots of times where I have relatively unique ideas. Very frequently people will not know what to think about them because they have never really explored that way of thinking. If I write in a way that makes it clear that I assume that the reader will find what I'm saying is obvious, I will get down votes. In a way it's insulting to say something that is not mainstream and assume that others will instantly understand/agree. Sometimes, if I feel the idea is interesting, I'll still post, but I'll try to broach the subject gently/apologetically. People are doing me a favour by getting out of their comfort zone and considering a different way of thinking.
Finally, when I'm posting, I often pick someone to respond to and try my best to help them. If I am successful, I find that the post is usually highly regarded by many people, even though my focus is only on helping one person. YMMV!