Honestly it depends on the person receiving the negativity more than it does on the person giving it. I doubt there's a universal way to satisfy everybody, but there are groups of people who can handle negativity better if it's interlaced with humor. There are also people who are able to recognize good will and ill will and base their reactions off of that rather than the negativity of the statement. And then, if people know you tend to say things for the benefit of others even if they are negative, people are likely to take it less emotionally.

For me, you have to know what kind of person you're dealing with in order to avoid emotional abuse. Some people are very easy to cross boundaries with and you should be super careful about how you phrase things, some people just need it presented in a certain way, and then there are people who have fun when they're criticized.

As far as the hard stop definition, I think the difference between negativity and abuse is intent, malice. Not knowing when to stop can be a very close second.