For each person, the answer is different. I'd say anything involving the following would rule me out --
-Hair pulling...like, really hard hair pullin'.
-Chains (well, wait, I change my mind)
(photo found here)
Don't get me wrong, though, getting pushed around in bed is nice 'an all. Even yelling is hot sometimes. But, if it hurts, that ain't cool...
Hell, what do I know?
I'd let someone cut my ass up if I was feelin' that individual enough. But, that would require some trust as well. Or, just a whole lot of sexual tension that needs to be let out.
Perhaps it's simply about what pleases you. Even if it means bleeding a little...or a lot.