My general algorithm for voting is to vote up that which I would have liked to have recommend for me to read and downvote that which I would be disappointed if it were recommended to me, where the criterion for wanting something recommended is does it thoughtfully engage with a topic in a way that advances my understanding (and in the case that my understanding already includes what is presented, I try to imagine the case that I didn't know what I know and vote from that place of counterfactual ignorance). I don't vote on things that either fail to pique my interest or that I feel indifferent on having recommended to me.
Strong votes (up and down) go to things that I would, respectively, be visibly happy or sad if someone recommended it to me, i.e. someone sent me an email about it and I light up and smile or frown and droop when I read the content.
In order of frequency:
-I strong downvote spam (weekly)
-I downvote people for antisocial behaviour, like name calling (monthly)
-I sometimes downvote comments that are obviously unhelpful or wrong (I'll usually explain why, if no one else has) (every couple of months)
-I occasionally downvote posts if I don't think they're the type of thing that should be on the Forum (for example, they're very poorly written, very incorrect, or offensive) (a couple times a year)
I don't think I'm a typical user, though. I largely agree with the boo/yay theory, based on my experiences with diversity posts.