I downvote whenever I feel a comment was made in bad faith; trolling in other words. This means I rarely downvote. If I disagree with the statement or believe an error was made, downvoting doesn't seem too helpful as the person will likely have no idea why the downvote was received. I'll sometimes even upvote what I think are errors as an interesting error that provides good discussion can be quite valuable. If you never make a mistake, you're not pushing yourself enough.

I've noticed more controversial topics tend to receive more downvotes even adjusting for quality of content.

I've wondered what percentage of the community hands out the majority of the upvotes and downvotes; especially downvotes because of their rarity.

A good chunk of karma votes are just pack following.