That is a good question. I think there is a difference between wanting to be submissive and giving up control versus being subservient. What little I know and understand about Femdom seems to combine both.
Giving yourself over to someone else sexually allows you to experience sexuality beyond your normal limits by giving over control. Wearing a leash, eating off the floor, being used as a footstool is a whole different thing. If the latter leads to the former, it is part of the "letting go" process. If it does not lead to a sexual element then it is just subservience.
Personally I like the idea of GFD far more than standard femdom. The slow and steady approach to giving in to her is much more arousing to me personally than what comes out of most femdom porn.