Usually when you think of pony or MLP fan art normally it is all cute and bright/sunny. I like the way that you put a dark twist on your work - it is a nice change from all the cutesy type of things I see (not that they are bad or anything like that, but understand it gets sort of nauseating).
Now regarding the actual way it looks: I like how you kept the focus on her eyes and the background (with the moon and such). It doesn't look scary, but I like the whole dark feel. There are really only two things that don't look quite right to me: her arms (or legs?) look a bit longer than her hind legs, and her wings look like they are down too far. Those are small things though.