Basically, a downvote is appropriate for any reason the user deems necessary. The main use of downvotes recently has centered around using them to police content receiving rewards far outside the norm: somebody posting a single picture with a sentence or two getting rewarded with $40-worth of curation (an outsize payment for such limited work), or to counteract the use of bidbots or whale self-voting where the user is seen to have artificially inflated the value of their own content beyond a reasonable means.

I've had content downvoted before for no reason at all, by the self-proclaimed Anti-Steemit Defense League, by people who disagreed with what I wrote, or sometimes by accident. This new HardFork gives everybody both an Upvote pool and a Downvote pool, so expect to see the downvotes flying now that people can do a thumbs-down without sacrificing their ability to earn off an upvote curation.

Just the way things are going to be around here for a bit, I imagine. :)