I absolutely love this picture. It's so suttle, yet so powerful. I love how Luna, especially her starry mane, really blends with the background; it makes it seem as if she's almost part of the sky herself. I love the color choice that you used, too! The blue and purple really compliment eachother nicely! Shading is excellent, too.
The only suggestion I really have is to maybe add a few more stars to the background because they're a little hard to see and take away a little from the starlit aspect of the night, which is why I gave 4 1/2 stars for impact.
It's still a beautiful piece regardless, and is one of my favorite Luna pictures! You have one of the best soft-brushstroke styles!
My first impressions when I see this Luna fan-art is that there isn't much contrast. It didn't caught my eye to looking at this one. So there is something you can improve so that the audience can gain interest by glance. For this one, I was not interested into looking at this one. I'm very sorry...
But now since you asked me to critique this, you might think of "What do I See?" And every artist or people looking to art should ask themselves "What Do I See?"
This is quite unique and very well executed because I think that the Spirit of Luna is the Night Sky. Her Mane shows the beauty of the Night Sky full of stars. So fascinating. That is the only thing I like about very much about this, Luna's beautiful Night Sky Mane.
The background is bland and this is the reason why I glanced through this artwork and look for something else. The color choice just almost coordinates to Luna's coat color, it makes Luna to be visually not there. Luna looks like a ghost (and she is an Alicorn). If the background was more black, there would be much more contrast between the Night Sky and Luna. Also, it would make the mane stand out more.
The anatomy is fine, except that the wing is a little too small.
I think there is lots of room to decorate here, but that is my opinion. It just needs more emphasis on the artwork itself. That way, it catches my eye and be more interested into looking at this.
I hope this will help you become a better artist!
Good ------- - Luna's Mane Design - Anatomy - Shading
Bad ------ - Background is bland - Not enough contrast - Uninteresting at first glance...