Derpy the Mailmare!

Mkay, so for starters, this was very well executed and in technique and st00ph I can only see 3 things to criticize (I'm a pretty harsh critic so that's not very many for me). One is about the lines in the tail, when they come together they don't flow smoothly together and a few curve in odd ways.
In the foreground wing you have 3 layers of feathers, but the layer closest to her face (or closest to the top of the wing) the feathers are at an odd angle, this creates a lack of free flow. The best way to fix this is to angle those feathers to point at about a 45 degree angle towards the tip of the wing, or you could just exclude them since most people only use 2 layers.
The last thing I am able to criticize is the actual image quality of the sketch lines which isn't a knock on your skill level at all. The lines are actually too grainy which can result from a variety of technical issues when actually scanning in your line-work.
Now for my explanation on my scores. Vision and impact would have been higher if not for a lack of originality. I see people draw derpy with letters flying everywhere on a daily basis. It's not an original idea at all. That being said, with the thousands of pony artworks being uploaded to the internet daily It's pretty hard for someone to come up with something original.