So first of all you did a great job on the legs and arms; with the exception that the arms seem to be carrying the staff in a unnatural way. Don't get me wrong the arms are the right size and bending in ways that they can it just seems that it would be very uncomfortable to carry it in that way.
You also did a great job on the wind it makes the whole thing look a lot more epic. Like she just finished a battle or something.
the horn seems a little bit small and sharp but not way to much so. The ears are a bit big they appear to be over 1/4 the size of her entire face. Ponies do have larges ears, but I think its a bit much.
The background is very nice and so is the foreground; however the... mid ground? the background that is brown seems without feature compared to the other two. I would just add some shape and shadows to it and it will be perfect!
Lastly the um... well.... Gosh dang this part is awkward to talk about... The breast seem a bit to low on her body.
Twilight is most magical pony ever, even when she's not a pony. And no matter her outfit or even shape, with certain limitations, I still love it. Those strands of hair in front look great, and the mane and tail are quite beautiful. Well done.