(This critique is all under my point of view and might not be as well...defined as others.) You did a fairly nice job with defining the stockings/socks, with wrinkles and where the seems lay. But her hind legs and hooves seem weirdly angled. I say that the shading and blush weren't that much of a bad choice, but her mane is lacking yellow (As it is a rainbow coloured mane). The tail colours seem to blend in nicely with each other. Lastly, the wings look decent but look like they were placed onto her, instead of being a part of her.
"Wow this is a really great picture, Complete with socks! Wait... What is that on her head?" Seriously I love this pic, but the bow, in my opinion just ruins the entire thing. Everytime I look at it my eyes are drawn to the bow! Am I prejudice against bows? Maybe... But that isn't the point. The bow is in my opinion awkwardly placed, and a bit small for her, Other than that the picture is AWESOME!
The first thing that comes to my mind is "Rainbow Dash always dresses in style." (the line about Rainbow Dash from the G3 opening song, I believe. Never saw G3 myself, but images and videos making fun of how different G3 and G4 Rainbow Dash are surface often enough.)