Of all my friends, I believe Rarity's hair (no offense to her, her hair is gorgeous, it's just a pain to draw and shade) is the most difficult to draw. Although if you do it in enough detail, anypony's hair can be a nightmare. One of my least favorite things to do of all things? Design dresses. That's why the only thing I do is copy stuff from Scootaloo's stories. She's MUCH better at designing stuff than me. Regardless, the drawing's awesome (and adorable).
I find that Rainbow Dash's dress is flying uncomfortably high. That's not to say I don't like it; in fact I'm in love with the idea of Rainbow Dash as an angel. It's a great concept and it's got me reeling with ideas. That's always the best kind of art! Great job.