Negativity isn't the problem--gratuitous negativity is. By that we mean negativity that adds nothing of substance to a comment. This includes all forms of meanness.
That's enough for reasonable people to get the idea. Trying to force precision here would take us down a rabbit hole.
I'm suspicious of the intent. That's why I want a more formal criteria.
Niceness/meanness is more often than not entirely subjective, relative, and arbitrary. Trying to create an objective definition for a subjective concept is going to end in frustration for a lot of people.
Personally, I started frequenting HN mostly because the community was not only smaller, but also more up-front about its opinions. I've come to expect and appreciate a blunt, direct criticism from the folks here, and would hate to see even the harsher end of that go away. The Theo de Raadts and Linus Torvadses of the world say "mean" things sometimes, but 93.8% of the time, those "mean" things need to be said.
Reasonable people can disagree about the boundaries of most things in the existing guidelines. But the guidelines aren't intended as behavior definitions for a computing system, there met as guidelines to be applied by humans to a human community, so fuzzy boundaries are normal, and trying to create excessively crisp boundaries for that task is usually counterproductive.