Atmospheric, but a bit empty.

I'd suggest adding a few clouds, the sunset kind, long and low. You could use them to hide some of the transitions, although I agree with DB you won't escape some hard work there!

I'd also suggest to shift the hue of the blue tones: towards red for the lightest, towards purple for the darkest, if you feel like realistic. Or any other way if you don't.

The tree looks too symmetric and even to me. And how about someone sitting under, or anything else?

If you think the (well anti-aliased) siloutte took time you're not gonna like what's in store for you concerning the sky! A large clean color range like this can only be "un-choppified" by extensive dithering or texturizing. Some artists might be tempted to simply use some paintprogram built-in range-dither (usually some form of the ordered dither), but that is not really considered true art. Here are two images of skies that uses a mix of dithers, textures and clouds to an amazing result:

pixeljoint.com/pixelart/23660.htm

pixeljoint.com/pixelart/33722.htm