Good tonality and lighting but the pose, to me, looks like a disembodied head balenced on another body's torso. Because her neck is hidden and her shoulder turned (tucked), the head no longer looks right for the body! Kinda disconcerting to me.
I think this is a very well done photo, as a couple of others have mentioned you have achieved some nice tonalities and the lighting is pretty damned good. I also like the composition.
Like rwyoung pointed out, the head looks very large in comparison to the rest of her body, and it's as if her neck is dis-jointed. Maybe that's just how it is, perhaps your model's head is really large, but it stuck out to me immediately when I viewed the photograph. Perhaps I'm being critical, but that was before I started enjoying the beautiful line of her back, how gracefully her hair falls on her skin, how the look on her face is playful and almost daring.
Perhaps using a longer lens (I think your mamiya can have its lens changed) would help compressing the composition enough where these things don't stick out as much.
I would say it's mostly a wonderful photograph, with a couple of things that could be improved. With that said, I would have been proud to say that I produced something this beautiful.