Hey Adam!
Now lets start with saying that this is an extraordinary event for me! my favorite animator steps into the world of digital painting!... so i'll teach you everything i know!
Now, Design and compostion is pretty much your area here, ill say that, and probably wont make as many critiques on that as i would on the rendering and colors bit.
now lets start with the robe and the cape, i like the swosh and the swish alot! but the colors and the rendering is a bit cartoony. my art teacher said that every pixel counts in digital painting, now she was a bit weird, but its been burnt into my mind ever since. so if you're going to finish the piece, have atleast 2 - 5 pictures of robes in the wind at the other monitor. ( i guess youve got a library full of refrence? ) also the little witch was pretty much consumed by the dark trees in the back. id bring here forth with some pink fog behind here.
Colors. they are really desaturated. except the stars though, which are nice! but id brighten up the witch's clothing alittle.
composition, as i see it the composition isnt at harmony, by placeing the biggest part of the cloud on the right side along with lemonee wee, the other side feels lighter, and my feel over the picture tilts little to the right. anyways i placed a moon there, to sort of get the balance back!
after writing all this i made a paintover!

hope you like it, and hope you wont ban me!( and oh, btw, if you're really serious about becoming a great CG painter, go over to Conceptart.org, make an account, and hang out there for a while, post in the critique center section and get alot of proffesional help. )