Upvoting: If I find a comment thought-provoking and well argued, or if it gives interesting, insightful or funny information, I upvote it. Occasionally I also upvote previously downvoted posts if I think the downvote unwarranted. Comments that express my opinions also often tend to get upvotes (if they represent a meaningful contribution to the discussion.) On the whole I am quite liberal about giving out upvotes, though I do take care not to do it in an inflatory manner.
Downvoting: I can't yet ;-) If I could, I would do it for comments that are way off-topic, contravene the guidelines or are gratuitously negative (especially if they involve personal attacks on other people in the discussion).
One thing I cannot stand is downvoting because you simply disagree with an opinion. We need diversity of opinion here, and on more important subjects than whether JS or Ruby is better for web dev. (As it is, the HN community is already pretty strict about enforcing a certain mindset on commenters. That is a Good Thing where values such as "Be polite" and "Be constructive" are involved, but can quickly become very dangerous if we start codifying opinions.) Basically, downvoting because of opinion pampers to confirmation bias and should thus be avoided. Note that I consider upvoting for opinion fine, as the psychological effect of an upvote is significantly weaker than that of a downvote - at least in my experience.