I think the strongest parts of this piece are the lines of action. All four legs, her tail main and wings flow together and away in a very convincing fashion. The sense of motion created by this is another perk. The shading is very good, so is the color palette, I like that you didn't make the butterflies the same pink.
I wish more attention was paid to the bushes. I know it's just back ground, but so are the clouds, and compared to them the tree line seems under cooked. Her eyes seem like a 'pass' as well. It does an okay-ish job of showing her emotion, but unfortunately does a better job of looking like you were avoiding drawing eyes. Not unlike drawing a character with both hands shoved into their pockets.
When it comes to fan-art, I’ve often given credit to images that are able to capture the feel and look of the source material. Here, while the colouring matches the look of the how well, we clearly see that the artist has used their own style to make the picture. But that is in no way a criticism as it’s a very appealing style. It’s dynamic, gives it a distinct look and makes the whole image worth looking at if for no other reason than it looks good.
The art itself shows a great deal of talent, with the aforementioned colours matching the look of the show when it comes to Fluttershy herself, but the rest is also coloured well, especially the clouds in the sky which almost look real as a result of the shading. But what impresses me the most in this picture is just how much everything seems to flow. From her wings to her hair, Fluttershy looks like she’s almost in motion despite being a still image, and that’s a very hard thing to pull off.
In terms of originality, the picture suffers, because I have seen a great deal of images on this site featuring this character in this situation. But, as I’ve said in previous critiques, originality, or lack thereof, is not an automatic bad thing for me. Just because someone’s done an image like this before doesn’t mean someone else can’t make it good as well, and this picture is proof of that.
Overall I’d say that this is a very impressive picture. While it may not be as complicated or striking as other MLP pictures I’ve seen, it’s still both well-drawn and incredibly sweet and charming. It’s a pretty picture made by an artist with a great deal of ability to them.
There's a very nice and delicate tension here, between the shy and retiring creature that we see in the show, and the fierce joy in this picture.
But you make it work. You make it seem completely believable that sometimes, when no one's looking, Fluttershy gallops through the fields, full of a happiness that is defiant of and triumphant over all the sorrows of the world.