Absolutely gorgeous and super majestic, that's what I thought at first sight. You've done a terrific job portraying my second favorite princess (after Twilight) in a simple yet striking way. I love how soft her expression is as opposed to what I usually see around, drawings that portray Luna as either seductive or bored. This one looks approachable, like a friend, I guess (, but a friend who's so much more beautiful than anypony else in Equestria) I like that she isn't wearing her crown or breastplate/necklace, adding to the Friendly Luna feel. I also really love the night sky effect in the background that blends in with the mane, flank, wings, and pool, it adds to her character as the Princess of the Night. It really shows how the very essence of Night is a part of her.
What don't I like? The head looks backwards. But that's not important.
Overall, Great Job! This drawing's AWESOME! I honestly love it and can't wait to see more