the back of Sunset's neck is a little too straight/rigid where is should have a slight concave curve at the base (it makes the eye look disproportionate and close together this way)

you did a really great job with her mane - all the flares and small bits really conveys the motion.

I like the lack of detail in Celestia... or rather I like that there's a lot of detail for less-defining features (wings, ears) but her face has no eyes and her hair has no lines for the colors.

The background is nice, the stars' alignment/proportions give it a vortex texture and let me know it's a moon we see Celestia in and not a tunnel

good stuff ^__^