Not full S&M mind you, just a *little* stimulating pain during sex: hair-pulling, light spanking, slapping, pinching, nibbling, etc. I find the occasional hair pulling or smack on the butt a great turn-on. It's just that extra unexpected spice!
Choking never even occurred to me to add to the list, though it is definitely along these lines. We both have allergies, so I don't think being unable to breathe has ever been a turn on to either of us, as it's difficult enough normally. Especially this time of year. Also the wife has a phobia of drowning... :S
Also: I love nobody's checked the "No pain for me" box yet!! (hahaha)