I agree with most of the posters that just because another may consider some behavior extreme, doesn't make that behavior abusive. However, I do think that abuse can occur in BDSM relationships. Just because you've consented to one thing, doesn't mean you consent to everything that follows or that you can't withdraw consent as necessary for health and/or sanity.

That brings up my next point: There are some who submit beyond their ability to maintain perspective. Their movement to the next stage isn't so much agreement or need or desire but, rather, based on the fact that they didn't say no last time and don't know how to say no this time. Whether from how they are wired or how their previous partners have molded their behavior, some people do get in over their heads. Permanent body modifications (like with scars from cigarette burns) need to be well discussed beforehand and care should be taken to minimize infection with antibiotic burn cream and clean dressings as part of aftercare.

I think informed consent is the main difference between play (no matter how consent) and abuse. Further, consent must be reached before play starts and pushing limits when your submissive is half out of their mind wanting to cum is borderline abusive.

Speaking from my own experience, when I was just starting out, I was pushed far beyond my limits by my first Domme. She used my body's natural response to stimulation to guilt me into believing that I enjoyed what was happening (ie "you wouldn't have climaxed if you didn't like it.") Luckily, I had a support network to help me out of that situation and teach me about limits, safewords and safety. I just worry that, even with the availability of online forums like this, there are other newbies that don't learn the lesson before they get burned.