In my opinion the very best fan art is fan art that you can easily feel and understand regardless of whether or not your a fan of source material and this is one of those pieces of artwork.
The darkness and shading set up a great background and absorb the viewer into the mood of the picture and Rainbow Dash herself perfectly sells her solemn, crestfallen, look in the spotlight with sadness in her eyes, her head hung low, and perfectly selling that surgically reconstructed left wing of hers.
The only thing I would've done differently was add some more detail to her locker. Some more pictures or some brighter colors to give a compare-contrast look to sell the humiliation that Rainbow Dash will never be a Wonderbolt.
Will she overcome her downhearted state and prove herself wrong? Who knows? But this really is a great picture!