I'm going to put my two cents in, and that's that nothing get's me more aroused than a good story with a girl that really doesn't want whatever is happening to her to take place.
I don’t know about everyone else, but it's a hell of allot more sexy for me to imagine a slave that really hates the position she's in, but complies just because she has to do whatever depraved sex act is on the agenda or face punishment.
It might be just my personal experience in the subject, which is "no girl would ever want that to happen to her", but man it sure gets me aroused!!