I like the overall style, and while we don't get a lot of background on the two characters pictured in the show, I think exploring it is a good exercise in providing feedback to Hasbro. Hopefully it will keep us in ponies for a good while to come as they take some cues from pictures like this.
The color saturation in the background greens and yellows is very nice, since these normally get either way over saturated or washed out. They really make the characters in the foreground pop.
The very subtle touch of the magic aura around Rarity's horn and the bubble pegasus was a nice touch. (I actually missed the part around the pegasus the first time i glanced at the picture.)
My only real thought on the (very slightly) down side is the colors in the foreground. While it's very obviously Rarity and Sweetiebelle, the amount of red in what should be pristine white coats is just a little odd. I understand that some shading is necessary for depth, I think you may have gone just a bit much...
Don't be discouraged just because someone else can do something you can't. It works both ways.
Example: I can't carry a tune in a bucket to save my life, but I (surprisingly, given that bit about not being able to carry a tune) make a good mixing engineer. My friend, on the other hand, can play 3 different instruments and sing, but can't run a mixing board even if I gave him step by step instructions.
Sometimes I think it's God's way of making sure we go make some friends.