Um... for me, I'd say anything is ok physically more or less. I think there's a line for me with things that I've heard some people like, like taking a cheese-grater to the genitals of another. That, I think, is something I'd like left to the cast of Hostel. But someone made a mention of face-slapping/punching. I'd be totally happy with that if I knew it was making my Dom/me happy. Likewise the deep-throating thing would be fine. Verbal though, is a different matter. As long as it was play, calling me pathetic, small, a slut, what ever, is ok. In fact, being called little makes me feel safe as it emphasises the size of the Top which to me infers power and thus protection. Areas that I'd find uncomfortably humiliating would primarily concern my weight, and my appearance. Being called fat, or ugly. Also, demeaning sexual references that play on my insecurities would make me sad. I think just like anything else, anything that pushes buttons become no less sensitive in context.