TwilightSparkle

The mane should be a flat line down the neck, rather than bulging out like it is, and the base should be broader, and come to a more defined angle at the base point. Bring the split up a bit, the mane there also should be on a single line, you can almost bring an arc from the top down over the split to the base point. Also, the MLP style has very smooth legs, so the knee shouldn't be a visible structure on the front forward leg (at least, not in this pose). The Cutie mark is .... way to symmetrical. The main star should be rotated 90 degrees, with the top and bottom points being oh, twice as long say, as the four side points. the little stars (only 5) should be scattered around the center, with three on the right, and two on the left (i think) and the entire thing should be a bit bigger and lower on the haunch. Additionally, the back slopes to far down from the croup to the withers. You want it to be almost straight, with a slight rise towards the back.
You also may want to make the eyes flatter on the base, the horn lower in the hair so that it comes from the forehead, and flatten the top of the mane a little, it looks... off? might also be a little high in the front.

That's about everything I can pick at, hope it helps.