I downvote for people being assholes (trolling, bigotry, ad hominem, etc), spreading misinformation, or making comments which don't add to the conversation ("This.", "This is the way."), and rarely for anything else.
I upvote content that I find interesting, educational, funny, etc. I also upvote people for being polite and willing to admit to being incorrect ("Thanks for the information, I didn't know that", etc).