Coolness, radicalness, awesomeness, any word that involves "cool" is what I would describe this! I like the shading of the sunglasses, the hair, and the texture of the hair. The background is real nice, kind of like the 'radicalness' is coming from RD, so I find that nice. The way her hooves cross, that's nice. also, how you put a shadow-ish color at the edge of her body to emit a lighting effect. You got her cutie mark just right, also how she's gritting, it adds a bit a roughness to her look, classic Rainbow. In Other (cooler) Words, RADICAL!!!!!!!!!!!!!!
Oh, I like this. It's definitely something that Rainbow Dash would do, and that is definitely a great pose for her. The shading is absolutely incredible, flawless even. The pose is very good, although the crossed fore arms seem... awkward for some reason; although that may just be me since I have not seen many ponies with crossed arms. Rainbow's main is very well proportioned and looked incredible with the detail you have added. Same for the tail, except it seems a tad too dark.
I am not too familiar with how cutie marks "shift" when a pony changes his or her stance, but the cloud and rainbow lightning seems odd.
The background is awesome. 'Nuff said.
The only thing I may have added was a bit more of her wings, but it doesn't make it any less cool. Backwards hat for the win!