As a veteran of web forums, semantics matter. "Flag" carries a universally negative connotation, and 9 out of 10 people see that word and associate it with "I did something illegal." And so, people get all fired up over getting "flagged." It doesn't matter what the effective application is, it's the word that's offensive.

"Downvote" means just that. It's like leaving a 1-star or 2-star review on Yelp or Amazon. You're not saying the establishment you're reviewing is a fraud, just that you don't like it. Two very different situations, even if the effective application results in the same thing.

If someone is going to go to the trouble of submitting a downvote button for the UI, it might be useful to actually make it DO something, like such toggles do on many sites: When you downvote, you have a number of checkboxes:

Downvoted for (check one):
Plagiarism
Spam
Disagreement with rewards
Not tagged as NSFW

And whatever else might seem appropriate. With that added, it might be a useful tool for cheetah, spaminator and so on.