I've seen this shot before! :-) Love the composition. The focus could be a tad sharper and I think with a little selective burning you could make it pop a bit more. Nice capture.
Nice composition, nice separation of subject and background (although a brighter background probably would look better). The biggest problem here is likely the softness of the picture. People on DPC love tack sharp photos. I'm guessing if you were using the sigma at 300 you probably fell victim to the lens a bit. The auto on portrait also may have given you a wide open aperture. That gave the nice blurry background, but also probably cost you some sharpness on the subject. The only other issue is some slight overexposure (not a lot though). Actually, looking at it again, maybe it isn't even overexposed, I might just be getting caught up in the softness.
This is a nice picture, I'm just afraid that this will be voted down because of the size, you should use the 640 pixels that we are allowed to use in challenges.