(Sorry for my flood) The rare cases when I down-rated people was when I felt they insult me. (max 5 times ?) But it's way long time ago.
And honestly, I don't get why people don't upvote people that efficiently help ....
More in the topic, I rarely upvote the OP of a thread except if he/she asks something, describes a problem I have myself.
So quite a subjective way of vote, and I let the community add objectivity.