This is wonderful. I love the simplicity of the feathers and horn, reflecting her poor beginnings. I like the sketched look of her body. The tail looks a bit like a snake. I would have made it thicker, though having it as you to reflects her strict personality. The background is stunning, and again, I really appreciate the simplicity of the design. Extra or unbalanced colors distract from a picture. The symbolism is fantastic. She is at once pressing against the wall, desiring greatness, but looks and reaches up right, past just the horn and wings, searching for something beyond.
While it'd be nice to say that earth ponies have strength or some such this is dwarfed by the powers of flight or magic.
The use of the chalky, fake surface to me symbolizes desires that cannot be fulfilled. Many pictures on desire ignite an empathy within the viewer of our own unfulfilled wants. Success, security, things that we often worry about.
This picture, to me, does a good job of connecting desire to identity and this is why it cuts so deep. We always want to be a better person. In the MLP universe the 'better' can be defined by extra limbs, a horn or wings. This is not true in the real universe.
What makes this work powerful is that it connects us to this understanding in the MLP universe intrinsically. We don't have to think about it, there's no deep analysis needed to feel the raw emotions.
Looking at the rest of your gallery this work feels like a happy accident. You didn't intend for such powerful imagery from the start, but were quite happy to go along with it as it was created. Much of the work here is fine tuned, tightened down with intention. I have this same issue with my traditional works. Controlling your work is not a bad thing, and you've used it to wonderfully expressive ends, but it makes this kind of discovery all too rare.
It is a wonderful thing to recognize these happy accidents (if it is not one I apologize for my assumption) and try to try to figure out *how the heck you did that* after the fact.
To me, a happy accident comes as the coincidental culmination of a few things. Much of which is simply drawing without realizing it. Drawing as a response, with or without planning. All too often I become analytical or follow a known process.
It is when you let your knowledge be guided in tandem with your instincts (the correct ones) that interesting things happen.
This is all very general and hard to understand for the purposes of instruction and figuring out how to capture how to do this again. So I'll throw out some advice that has helped me in the past.
* Study the best. Much like reading gets you in the same mood as the author, studying artists and strong compositions gets you in the same mood as strong compositions. You absorb it like a sponge.
* Work on responsiveness. Your speedy sketches are an awesome way to do this for emotions, but perhaps speedy thumbnails would assist you in compositions?
* Read. Art is made as a response to real life, not in a vacuum devoid of other artists. The wonderful thing about reading and being an artist is you get these neat pictures in your head, then you can make them. It's... well, it knocks my socks off.
As with all my critiques, four stars for everything. Critiques are about a discussion and learning, not ratings.