Two words: Common sense.
The thing about lifestyle is that people blur the lines between fantasy & reality.
Walking the line in BDSM activities can be an absolutely amazing experience in some cases, but oftentimes without proper training, knowledge, or forethought, experiences that walk the line can turn abusive or disasterous.
Walking the line between fantasy & reality in writing can also be erotic for some, but the issue is that other people may read it as reality... take EVERYTHING YOU READ with a grain of salt & use that common sense check: does it just seem too ridiculous to be true? IT PROBABLY IS!
As for abuse... there have been more threads written on it, and more opinions given about situations then I can count. It really is a case-by-case issue, though, and any "cookie cutter" response is probably not appropriate for any one situation. Abuse is, plain and simple, wrong. If you think you're in an abusive situation, get help.
However....
Often in Lifestyle, some activities people would perceive to be abuse aren't to the people doing the activities; verbal humiliation, degredation, objectification, scarification... for some, it is how they are aroused. For others, it would be pure torture. You know yourself best... if you think it's torture, then don't do it. If you say "no," and your partner continues, it's abuse.
It's impossible to sit back & be an "armchair" therapist for anyone's situation. Why? Because we aren't there... we don't know the dynamic, the situation, the external influences, the people... so saying anything about someone without being there is like commenting on people you see walking through the mall--you are putting your perceptions onto the situation.
Just my two cents.