I downvote nodes which I consider to be stupid or wrong (yes, there's a difference) or where the author has apparently made no effort to solve his problem himself, or where the author has chosen to appear to be illiterate (by eg not being able to spell or punctuate despite clearly speaking English as his first tongue; but then I guess that's a subset of stupid).

I upvote nodes which provide useful information or which are funny. I especially make a point of upvoting nodes which ask interesting questions. Those last are, I think, the most important nodes on this site, as I often learn new ways of looking at problems from them, and new ways of solving problems from the answers they generate.