Very nice job. The lighting is beautiful and the sweep of the skirt alluring. I only wish somehow I could see the top of the straps so the dress would really appear floating on the black background. A small thing in an otherwise wonderful image.
Since this is a staged still life photo, why didn't you reshoot it to get the shoulders of the dress in the shot? I wonder if fashion photographers stuff the chest of dresses. Something about the sunken in chest area doesn't work.
Lacking a bit in sharpness. The lighting treatment doesn't really flatter the subject to its best advantage. I like the use of a black background. It really makes the dress stand out.