You desaturated too much of the dress while doing the other colors. You lost some of the R/G/Y color components that would have made the dress seem natural and real.
This is a very nice black and white shot, and IMHO the color in the dress doesn't add much. I think that rendering the whole woman in color would improve the effect dramatically.
I like the effect created here. I think that the subject isn't quite "interesting" enough to hold me for long, and a little more sharp contrast in the B & W portion may have increased the impact possibly.