I first want to say that what I admire most is your incorporation of Asian themes into the clothing. I know as your fan that you tend to do that in many of your artwork and original characters and designs, but to be able to put in on Princess Luna and Celestia and make it look very MLP shows a good aesthetic sense and being that much more creative with your fanart than others out there. Also, the way you skillfully displayed the cutie marks look very beautiful and natural. Celestia's huge bold golden sun reflects the burdens of her duties, being ruler of Equestria. In contrast, Luna's mark is smaller, more subtle, but equally regal like night itself. The fact that the size of the moon is much smaller than the sun is a good reflection of their roles in the show.
I like how Luna's outfit is much lighter and less ornate than Celestia's, who is weighted down with robes and jewels (again, in reflection of her great responsibilities). The creative take on the necklace is nice too, and pleasantly recognizable yet new. Same with the crowns, although understandably less garnished, but still has a solid, realistic look with the coloring you did. The personalities of both characters is evident in your design, and visually they contrast each other, but in doing so make the other look elegant in her own way. PROPS ON THE HAIR. The beautiful billowing hair of Luna looks much different from Celestia's gently illuminating mane, making Luna playful yet mature and her sister serene and bright. The hair is one of many ways you make this duo seem like sisters yet still they are distinct beings.
Now I've been showering you with praise, but here are somethings I think could be improved. Luna's face looks very horsey, eyes true to MLP style and genre, but Celestia shows a slight departure, looking more fairy than pony. This facial structure change is slightly confusing---in a way shows a crack, an inconsistency. The hands, ankles, and wings show a lack of attention, not displaying any detail that was given to the clothes and jewelry(especially the jewelry). They were drawn in a way that can be easily ignored, but to make them more definite and refined will make them as a whole that much better and that much easier to enjoy. The wings don't really have feathers. What are the hands? Hooves or hands or both? The fingers seems so hastily drawn, and stubby looking in an hesitant way. And the ankles look like kankles. Just undefined, unsure, hooves or feet?
Celestia's lower sleeve is playing optical illusions on me. Am I looking at the inside of the sleeve ore the side of the dress? It's confusing in an unsettling way. Please somehow try to clarify...
I only noticed at the very last second that you drew tails. I first I thought it was hair but it didn't look quite right. It just looks...off. One way you can clarify the existence of the tail is to make it much longer and have it trail around their feet, or swirling on the floor...indeed, their tails do that in the show.

Here are some suggestions that are just extra goody goodness, not entirely necessary. One, the white-grey backdrop is perfect for showing off character designs, but as a picture it would be great if they had a background setting, like strolling on the Milky Way? Another is that Celestia look a bit overly melancholy, which is out of character. However I don't know if you watch MLP so whatever.
Holy moopoo I wrote so much...and about My Little Pony too!!! lol well hope it helps on other artwork as well...

I'd better preface this by saying that this is written from a person who's seen one episode of FiM, and retained none of the information within. I'm not a brony, but I am a fan of the wonderful works of TysonTan, whose drawing of a tiger playing soccer rests happily as me desktop background. Just putting that out there.

Anyway, I'm not sure whether I like this so much. Knowing it's inspired by MLP doesn't really bother me inherently, but I do feel that it restricts creativity a little bit. Especially because you barely need to squint to imagine that they aren't ponies at all. Whether that's a good or a bad thing is up to the responder (but I like it).

Originality in context is good, but the My Little Pony thing is getting a little stale. I mean, search "princess celestia" on dA. How many results appear? Lots. But then, y'know, it's a unique integration of the human form (unique because it's well done and not unsettlingly over-sexualised).

It's always hard to fault this artist on technique. Just check out the care in all of the details. The masterful shading on (particularly Celestia's) clothing, the subtle but amazing textures that add depth to the image and the lovely colour choice all bring a lot to the work. My only gripes would be some evident peculiarities on Luna's foot/hoof, and that Luna's expression lacks the purpose evident in Celestia's.

Ultimately, this work doesn't hit me as hard as others by the artist, and I think that's because despite what I tell myself, the pony fandom bores me. It bores me because it drains creativity from artists all over the internet who have previously demonstrated their incredible creative power and funnels that creativity into a stock set of characters from a kid's show (yes, I know it's meant to have "intergenerational appeal", but it's called Friendship is Magic, how seriously could an adult take it?).

But then, if I wasn't told they were from MLP, and if Luna's face shape was a little different and maybe if they didn't have horns, there would be no reason for me to dislike it: it would merely be contrasting day/night characters, and it would be awesome. And that's what's got me in two minds. When you do fanart, you restrict yourself to a canon, and that can be exciting, especially when the canon in question is enormous, like you'd find in fantasy fanart. My Little Pony... eh, not so much. Does that make this a bad image? Definitely not, but I can't help but be concerned about where this is all going.