First thing that caught my eye on this was the amazing level of detail in the hair on her mane and tail. I really love the fact that each strand of hair is so shiny and visibly vibrant. Also very nice is the incorporation of realistic soft coloring that makes her look believably 3D while still retaining proportions true to the original art style.
As for the background, I really like the frost-covered trees and snow covered ground with her hoofprints trailing her that makes it look like she's enjoying a trot through a winter wonderland. I also really like the night sky and especially the ambient Aurora Borealis-like montage of colors in the lower atmosphere. Rarity's nearly-white coat was a little hard to make out against the pure white snow, but that can't be considered a fault considering the context of what's going on in this image.
If I could offer a critique, the clouds in the background are very nicely drawn and consistently colored and lighted, but appear a little too perfectly round and widely curved. The coloring method used for the eyes, while very true to the original source material, seems a little cartoonish compared to the breathtaking realism of the rest of the picture. Perhaps a softer and more realistic technique could be applied to the irises to make them better blend into this pic.
Overall an amazing piece of work and I hope to see more like this :3