Broadly, anything that I don't foresee being dangerous or which would, if conducted for other purposes, require some sort of specialized training/qualification to do. i.e. making an incision, of any sort, is a controlled medical procedure here requiring one to be an M.D. D.D.S. R.N. EMT-P equiv, etc. If I can abstract it to being similar to something that requires professional qualification and facilities to be conducted by the book in other situations, I'd generally leave it alone unless it happens to fall within the scope of my own qualifications and facilities available. But i.e. I don't have a sterile field, instruments, local anesthesia, etc...so I would refuse to do an incision. I don't have a ET tube+larynx-scope and ventilator at home, so I'd refuse to do breath play. Doubly true if I'm subbing..which I usually prefer, because then it's the other person's competence that matters.
Same goes for other things...it's NEVER acceptable to, say, point a loaded firearm at another person unless it's a situation where it's reasonable to suspect lethal force may be required, so I'd never feel it acceptable to use a loaded, functional firearm in any sort of play. It's a big no-no to leave a person in handcuffs face down on the ground unattended, cause of positional asphyxia so I'd never feel it acceptable to leave someone unattended while restrained in a similar manner during a scene.
Rope bondage- same deal...tightly applied rope can cause a limb infarct or nerve compression injury if left for too long. So I'd never think it cool to leave someone tied up with rope for a long time, and always monitor their limb perfusion during the scene (check for cyanosis and capillary refill time on a frequent basis) Holding a stress position for too long can lead to Rhabdomyolysis, which is bad and can be fatal or cause permanent kidney damage, so for the same reason, I don't wish to participate in any sort of extended scene involving stress positions.
I might seem overcautious; but I've seen all of these issues happen, both from non-sexual activities, and from BDSM gone wrong.
PLUR.
