Alright, I'll point out some features I feel are a bit off; and the things I like.
Starting with the 'bad' parts, -The ear of the RD on the front appears to be floating in the air. It looks a little bit small as well, judging by the size of the head.
-The wings of the same RD are very small compared to the rest of her body. They could do with a little enlarging.
-Similarly, the wings on the top-right RD are very slim. Their length is good, it's just their width that's off. There is no way wings that thin could ever support that body.
Now for the parts I like, -The shading is done nicely, and thoroughly.
-The colouring is very lively and good looking, I especially like the way you let the colours blend into one another, instead of having a clear outline for each.
-The grainy texture adds a nice little something to the piece overall.
Judgement: I think you could've done better anatomy-wise, but everything else looks very slick and well-executed. I, personally, haven't seen a drawing like this one before, so in terms of originality, I like it. The overall style in which you've drawn and coloured the picture is lovely; and really adds to the drawing as a whole.
I absolutely love the shading and the colours! It's sort of reminds me goals and dreams you have because of the smile on rainbowdash, who is daydreaming she's a [now it's slipped my mind thunder/wonder bolt?]wonderbolt?. You kept true to her hair style but in your own way, which I admire definately, the picture isn't too busy and you leave it open for interpretation (in my opnion) I LOVE this picture If she's not watching herself as a wonderbolt do tell i just assume because none of them have a rainbow after they fly except rainbowdash Fantastic Job