First off, love the creativity of this piece, I haven't seen too much of this kind of work.
Well, lets get into it...
Pros: - Each character is acting in character which is nice to see you have taken the time to choose the vectors carefully. - You have also taken th picture in a very nice location, and would still look amazing without the ponies in it. This is good as it shows you have put thought into all aspects of this picture, not just focusing on one. - I like the interaction Pinkie has with the background, it helps sell the idea that they are actually there. Since this is what these IRL pictures are all about, it is good to see you have included that.
Cons: - I would say that just on that last thought, you could place more of the characters "in" the scene by showing various kinds of interactions, this could include hoof prints in the gravel, or ripples in the water under RD and Fluttershy. - Another area to be really careful around in shadows and shading of the subjects. I noticed that you have put in shadows for Twilight and Rarity, but you need to make sure they are following the other shadows originally in the scene; i.e the direction of the Sun in relation the the characters. I can see that the trees shadow appears to be coming at the camera, whilst Rarity's and Twilight's are going away or parallel. Another note on the shadows, especially Rarity's, is that you need to match the tone of the trees shadow; hers outweighs the trees, and this can ruin the effect. Also just add a little soft shading under AJ. - Finally on the shading point, this picture has a lot of variation in exposure, and that can work to your advantage if you get it right. Here, around the water area, there is a very bright reflection off the water, and I think RD and possibly Fluttershy needs to be consumed slightly by this. Also as Applejack is in the shadow, try and match her shading to the tone of the shadow. If you do this, there will be a definite sense of realism to the scene, and it won't be as if the characters were simply 'placed' in.
Overall, I think the originality and impact of this work is fantastic, and like I said before, choosing the vectors carefully was done really well; but just ensure you allow the characters to interact more with the scene; hold a leaf, splash in the water or simply lean against the tree, it all helps .