I tend to upvote a post when it Links an interesting article or provokes an interesting conversation, and only downvote them when they voice an convectively bad opinion (like, bigotry or other unjustified vitriol).
I tend to upvote comments when they make a good point or at least try to support their argument, and only downvote them for the same reason as posts: when they are trolling, being unnecessarily aggressive, or trying to justify bigotry with fallacious reasoning or something
What about you?
However the mood strikes me. I upvote things that I agree with, enjoy, or otherwise approve of. Downvote mostly jerks, liars, and pictures I don’t like. Most stuff just glides right by.