I love the tenderness and almost vulnerability portrayed through her soft expression and pose, but the strength and beauty simultaneously shown in her flowing wings and the moon. Very difficult to balance well, but this is near perfect. Simply gorgeous!
This is a very good example of composition. The way you have Luna's wings curved to match the curvature of the moon makes the drawing more unified visually. And you've created a really striking effect by having the glow of the moon 'back-light' the figure while having most of Luna covered in shadow in the foreground. It looks like an animation still.