There's a beautiful colour to the light; warm and rich, it complements the furniture and the model's dress. It really does give the image a "Great Gatsby" feel; very elegant. I love the way you have her subtly backlit here, casting a halo around her outline and hair. It involves the loss of detail in the highlights on the part of the couch under her arm from where the light is hitting it, but it really doesn't matter because it's tempered by the more subtle exposure of the rest of the room and the way her right arm breaks up the pattern of the couch anyway. The last thing that struck me was the position of her eye; not looking at us, not looking away from us, almost teasing us to determine whether she's going to turn to us and allow us into her world, or turn away and shut us out. And the depth of field couldn't be better; it's sharp right up the plane of her back and her face, and no further. The loss of sharpness in her arms is subtle; not abrupt or distracting. I've noticed that the 5D III / f/1.2 50mm combo does this very well from other shots that I've seen. Another very, very fine piece of portraiture there.
