Wow, I absolutely adore this picture. The wings have such amazing detail and the clouds just fluff naturally. Her expression is priceless! I have a problem with seeing Dashie as a dude, but this is the first I've seen that completely captures her feminine side; mostly in her expression. I love it!
The trick is to have things be self-consistent, not physics-constist. Just assume physics work differently there but in a way that is still a self-consistent physics: a pegasus doesn't fly by lift but by being attuned to the air element; earth ponies make stuff grow (including rocks and jewels) not by virtue of being excellent farmers, but because they're attuned to the earth element; going with this I'd guess unicorns are attuned to fire what with their explosive power and levitating stuff (fire goes up) and all; and the as yet unseen seaponies go with water.
Discord is chaos because he doesn't follow any of those internal rules. He defies all physics: both ours AND Equestria's.
Excellent job. ^^ Your posing is very good, and I love that you put in a lot of effort when drawing the wings. The shading is a bit confused in a few areas, like the left flank and the left side of the nose, but otherwise it is quite good.