Well, lets start with this pic's greatest flaw - it has no Applejack

The concept here is very clear and quite striking with three ponies sleeping, while Luna, Princess of the Night, guards their dreams as she was set to do. The general spread of details gives it a touch of majesty, becoming all the more eye-catching.

Originality is always hard to grade; on one hand Luna has been almost overused among fandom; but, the ironic part is, I rarely see her in a setting befitting the character (unless we're talking about portraits). To that end, it's a nice change of pace to see someone playing real homeage to her.
The overall design has the feeling of uniqueness, even though such a scene is not particularly new as a whole. I, for one, find such a mix refreshing.

The technique is very eye-catching indeed; Luna virtually shines and glitters in front of the black, night sky, whiel Fluttershy, Rainbow and Scootaloo look adorable (if you ignore the mild subtext that some can trace in the pic) with their manes flying all over the place. All this is very stunning.
The background, however, could use some improvement. Overall it's very decent, but the lack of any stars in the sky takes away some magic from this piece. Another thing is the fact that some parts of it feel like they were done in a different style then the rest of the work; this is specifically striking with the meadow that surrounds the characters; it seems a tad weathered by comparison with everything else.

The impact is the most interesting of the bunch - while it is undeniable, it remains very gentle, sipping into the viewer only it the latter allows it to. That is a very unique outcome that I rarely see among fanart; on one hand Luna's majesty and grace is almost striking and yet it's toned down to a level that it wouldn't beam from the piece.
Similarly, the peaceful nap of the trio spreads across the borders of this work filling it with serenity, a form of beauty even, but also this is somewhat overshadowed.
The ironic part is that I find that very fitting for the concept, nodding to the calm nature of the night, when most creatures drift off into the land of their dreams, more often then not getting wrapped in fantasy, that is evident in this piece. Luna in particular stands strong, becoming the introduction to the whole as she spreads her wings, almost like she was inviting the recipants to winnow the rest of this pic with their eyes.

The impact of this piece may not be "strong", but that's not necessary a bad thing; often a more subtle message is more suitable and, undoubtfully, this is one of those cases; while some more detail in the background would've made this work even more stunning, it fare's very well even now... despite not having AJ