I think some examples might be helpful. I would consider a gratuitously negative comment to be some variation of "That's a dumb idea" with no elaboration, or an ad hominem, and such comments are pretty much always downvoted already.
Here's a few examples I know of: https://news.ycombinator.com/item?id=9015734 https://news.ycombinator.com/item?id=8938409
So I think informative examples would point out comments that many people think are okay but that the HN team does not.