Hir Charly.
You asked me the difference between your girlfriend (to whom you can do anything, but won't be your slave) and my subsletkatrien.
For me it is liberating to know that she is under my control, that i have power over her. She gave me the power to enslave her, and that's even more satisfying : knowing that she wants to be a slave, that she want's to be dominated. For her it is liberating too, she has not to think about what to do, or what not to do, she just has to listen to me. In a vanilla relationship there are also power structures, but not always that significant. It isn't clear in a vanilla relationship that either the man or woman is "the boss". And because it isn't clear who is the boss, there wil often be power-clashes and fights between the two partners.
In a D/s relation that is less often the case. And that makes it more satifying for both partners, she knows she hasn't power over me, but only has to listen. And now whe are not talking about sex! In bed , or sexually this power structure stands too !
You're girlfriend likes maybe "rough" sex, but is not willing to become a slave, maybe tha tis because she isn't slave material..:-) As in : it isn't because she likes to be treated rough in bed, that she is a real slave girl. A real slave girl is also submissive out of bed, she just is a slave always. Sometimes she won't show it because of the environment, but she stil is....

Good luck !
BaasGeert