Love the lighting and quality of the photo, and the quality of the costumes is great, especially in that not one outshines another. All of that contributes to the piece but none of that is the reason why I consider this piece superior to other cosplay photos.
Candid photos, where the model is posing for the camera, are the norm when it comes to taking pictures of cosplay, but more often then not it's a portrait of themselves. This photo shines because it's not a portrait or just a group photo. There is a key part that this photo that most other photos clearly lack - and that is story.
You can ask yourself, "What are they doing?" and give a clear definite answer. Everyone's reading while Twilight is looking pleased at a party of some sort. This is something that is completely believable in the world of My Little Pony. You can see this shot being part of some movie - and that is why I love this.