I couldn't be anymore impressed with this piece of artwork. It feels like you can feel Luna's pain, hatred, and jealousy of Celestia and her "Precious light" that everypony loves more than her beautiful night. This is something that you would see from the "Lullaby for Princess" music videos on YouTube.
The only problem I have about this is her mane. I know you had to make it big enough so you could fit the reflection of Princess Luna and her angry tears in Nightmare Moon's mane, but it's a little too big.... Even for her!
But it's still cool though