I don't subscribe to the SSC theory I prefer the RACK one. For me the big difference is consent as some have said and the intent. His intent is to cause me pain but not to do permanent damage. It is a fine line. I don't agree with the unfulfilled part because abusers to feel a sense of fulfillment and power. Also you can find a myriad of psych articles and studies that say abusers do care but have this ineffective coping that makes them act out in an abusive way. As you said the abuser feels superior, that is the abusers fulfillment so that is a contradiction.
Abuse can be anything that, that person views as it is. There isn't a set groundwork for this is abuse and that isn't. It is a fine line and a subjective one in BDSM.
Watch this video it is short and very informative. From a psychologists point of view.
http://www.youtube.com/watch?v=uzNd6TIeexU
Abusers isolate and create anxiety to keep the victim compliant. Here is an article which is very good at explaining abuse.
http://www.googobits.com/articles/p0...d-to-know.html