I'll go ahead and say this right away: This piece is GORGEOUS, and I can tell that you put an immense abount of time and effort into this. So very well done. Your background contrasts rainbow dash's colors well, but it might be a good idea to make it a little lighter or or darker, as it does blend in with her armor a bit. VERY good amount of detail on the eyes, armor, and the mane especially. You also did very well with the lighting and dimensional aspects. It makes the piece look very realistic.
Very very good work. The only thing I can possibly think of making a suggestion for is the backgorund. I applaud your work. You should be very proud of it.