I then realised that the remaining up arrow could be used as a `I've read this post' indicator and started clicking all of them as I read old posts. Very useful to help pick up reading where I left off. Funnily enough, I'm not aware of anyone complaining that they'd got lots of up votes over the period I was doing it. :-)
Once I realized that voting up was the best way to keep track of stuff I might want to come back to, I used it for that, even though that means voting up stuff only I would care about and not voting up stuff that might be useful to the larger audience.
Comment voting is a little more interesting. I vote up comments I think reflect an interesting point to the discussion, even if I disagree. But I know that lots of people vote comments up or down solely on a basis of whether the comment agrees with their pre-conceived notion of reality.