Something that feels like it could be improved is how her tail rolls into her rear, that is, it needs to bunch up more at the base.
The other thing that feels off is Angel doesn't have enough cast shadow in comparison to Flootershy. Or any occlusion where his body meets the ground. If I just squint my eyes he pretty much blends right into the BG while Flutter still pops, so there's something a bit inconsistent thar.
That's about it really; there's other tiny, tiny nitpicks like her wingtips could be more feather-like (fff, like MLP pegasus anatomy is supposed to be accurate XD) and her front shoulder could be better defined. But really, none of those issues takes away from the emotional impact of this. NONE. :3
Probably if you submitted work like this in a studio setting, the art director would tell you this and make you fix it, but really it means you're competent to keep going. ;D
I love how this shows her character. The lighting and the shadows look excellent and expert. The color scheme is really pleasant too, like an eye massage. (But her legs look weird... are they supposed to bend like that?)
Beautiful! The whole atmosphere is like something out of a story book.
The composition is great. I love how Fluttershy is framed by the trees. The shadows from the leaves are really well done and make for nice, interesting patterns. I also like how you exaggerated her features. Really nice work.