Pinkies look rather off just hanging there, the ratio of her hips to her torso seems a little weird as is Twilights eyes and horn (But that's just being nit picky)
Over all I think it's a very good drawing and I wish I had half the talent you do although I'm not sure if I've seen this picture before so that's why the rating for originality is lower then is should be.
And lastely this has nothing to do with the picture but a 100 word minimum review is a little excessive, hell, right now I'm just padding it out so I can sumbit
Ok, first I love that you combine Twilight, Pinkie Pie, and Derpy together as those aren't an average combination from what I've seen.
Background: This piece is set on a really cool background of realistic clouds that are soft and airy. I find that really cool and well-done as clouds that are realistic can be hard for someponies to do. That being said they don't really fit the classic MLP:FIM type clouds as much, and those might have made this a little better to match the show, but that's just a personal bit.
All ponies: They need to have a little nostril showing which I cannot see. Body positions and dimensions, as well as proportions are above average and pretty well done (with the exception of Derpy's face, see below).
Twilight & Pinkie Pie: Their colors seem a tad lighter than desired on their coats, their tails and manes are fine. Their eyes could use some more defined eyelashes as well as a little iris color for twilight (just a little, not to remove the shocked effect you're going for). Twilight's tail could use a little bit more gravity pulling on it.
Pinkie's Balloons: The balloons are cool, and the realistic translucent and transparent elements make them look like balloons, but their edges seem to be a little bit off, I can't tell exactly what it is, but there's just something about the edges.
Derpy: Having Derpy sleeping up there in the clouds is very cute, but she needs to have a discernible snout. That being said Derpy's face is more rounded, but it still needs a little snout to really cement that Derpy look.
Color Selection: All characters are done pretty much as close to the hues that are in the show and are pretty well done.
Overall: This is a fairly decent work and is certainly above average quality. It's a very summery picture and surely is a good addition to the group galleries it is in.