Assigning a ++ or -- for me is the sum of a number of things:

• Is the post useful/informative or is it a waste of time?
• Has the writer (whether posting a question or an answer) done a little research, or is there no visible effort?
• Do I strongly agree or disagree?

If I can't really decide one way or the other (which is most of the time) I don't vote at all. I rarely get through all of the votes I'm issued in a day.