I hope you won't think it's crass of me to say that that is a perfect ass!
The picture as a whole is a wonderful piece of erotic art. I love how the wave of her hair echos the curves of the backdrop. And your use of contrast is masterful. I noticed how none of the various tones in the backdrop really match the tone of the model's skin, which draw's the viewer's attention on to her. (As if that gorgeous deriere wasn't enough incentive!