For some reason, this picture reminds me of Children of the Night.
It's always nice to see pictures with the Princesses being more leisurely, kicking off all that regalia they always seem to be wearing. It gives a sense that they're just relaxing and not having to worry about their usual royal duties. Maybe there's something about the night that makes it easier to make such serene images.
I do have to wonder why Luna's mane is a mix of her season 1 and season 2 styles though. Did you initially want it in season 1 style and then switched later on and forgot to remove her mane? I also find it curious Luna is resting on a pile of hay, rather than in her bedroom or something.
Luna is very rarely seen in such a style. Her hair seems disheveled, and her figure is overall much less Alicorn-y. The eye lines have a curious two-fold effect: They remove the veil of "Perfect Alicorn Princess" and they remind the viewer that Luna is actually thousands of years old. The view is also unique, on a rooftop of a nondescript barn or commoner's house.
One thing that could use work is on the background. The style isn't quite as clean as Luna, or the starry sky. I would almost suggest a third-party background behind the immediate rooftop that Luna is on.
I'm also a stickler for show style. Usually, I'd comment on the snout, as the royals have longer, thinner, more ladylike features. In consideration of the attempt to lift the facade of a perfect goddess, however, the "commoner" snout makes sense.
The originality is a little marked down to me as well because, "oh jeez, its another Luna staring at the moon" was my initial reaction when I saw the thumbnail. The full picture blew me away, though!
This is really an outstanding picture. I love the small touches, such as the "z's" floating up from the chimney. As I said, the drawing does not show luna in the perfect, royal state we are accustomed to. This has both positive and negative results.