Very good job here! The dress and outfit is very show-accurate, and the lines are very clean. The outline of the eye is very clear, but the eyelashes could be thinner at the tips.
The coloring is very smooth, and I can't see any white spots where there shouldn't be. Some spots of the dress appear darker than others, but then again, getting a cover as clean as yours isn't easy! Some of the stars on Twilight's saddle could use some *minor* improvements, but I must say that you've done an extremely good job, and your coloring skills are terrific!
For originality, you could have added a more complex background, and maybe drawn her in a more interesting pose, because this one is fairly common. Why not try drawing her drinking something, or dancing? I know she didn't really do either of those in the episode, but then again, use your imagination when drawing! There is no "right" or "wrong" when it comes to art.
Overall, you have done a brilliant job with this piece, I love it! Keep up the good work, and remember, use your imagination!