Simply. stunning! Honestly, those eyes just stood out to me! and her mane... oh don't get me started on that! The eyes are a little big, but that's your style.
Princess Luna, is like, the best Princess ever! (other than my OC ) at first, I didn't reconize that this was Luna, until I took a closer look, and read the title. I really like how the colors go from light to dark! I have nothing else to say! XC BT*N^%BY%Y^NK*(*OO>PLUNA IS BEST PONY! HUZZAH! DID I EVER TELL YOU I LOVED THE EYES? lol, this is amazing. you, my friend, are the best! THE BEST.
I really enjoy your color choice for Luna, and that she looks truly pleased- though what her pleasure is- is hinted at by the crown. Its a bit of a mystery. Did she take it? Was it given? It is very thought provoking, and that is wonderful.
You light-source and direction are good, and the hair is both indicative of filly-Luna and the nimbus of adult-Luna, which is something that is rarely executed well, and was done so here.
The colors between the background and foreground are a little too close to make easy distinction, particularly around the ear. I would suggest adding a little more purple or cyan like her eye, just a smidge, to help differentiate the colors in addition to the texture.
Luna's hair is soo sleek!! And her eye...that color just pops out at you when you first look at it. And, for some reason, the clouds are one of my favorite parts of this piece. You would think that it would take the attention away from Luna, because they would blend, but it doesn't...I don't know, it just seems so beautiful to me.