Well, I'll start by saying this is certainly noteworthy, it has lots of things going for it, and your skill is quite obviously there. There is room for improvement, but that is something everyone has, and I can see you making some truly great works in future.
The style of the piece is very Rainbow Dash. Instead of the high-riding dress and the clunky shoes, this new style is much more speedster, and as minimalistic as one would expect from this character.
The shading was for the most part, awesome. You segregated very well, the costume and the actual pony aren't molded, like a lot of pieces I see. The tail is kind of plain, but I attribute that to the tail band, which would tease the shape that has.
The detail is really quite beautiful, her mane & tail are incredible, shaded, lined and segregated very nicely, and her wing is very good looking.
I suppose my biggest problem with this was the wing, it has to be said. It looks very plastered on, in comparison to the rest of her, and in my eyes, the piece suffered for it.
The next small problem is the cutie mark, while it certainly looks detailed, it is too one-dimensional for her, it looks slapped on instead of sketched on, I think maybe less care was taken with this bit?
The last problem is the eyes, they are very good looking, but the shading is kind of against the style of the rest of the piece, the glow-shading seems more like a spur-of-the-moment thing than the planning from the rest of this. This is probably more of an aesthetic thing, so you can disregard that if you wish.
All in all, this was a rather good piece, and you have quite a lot of skill there, but it needs to develop some more, and you need a more consistent style. I love this piece though, 8.5/10.
Thank you very much for your critique
You are totally right with the cutiemark. It actually is just slapped on, taken from a previous picture.
The problem there is, that I uploaded the picture, without noticing that I forgot the CM. So before anybody would come with a comment like "Blankflank" I just edited something quick in and changed the file.
If I find the time, though, I'll definitely redraw it