Aside from these great reviewers (keep them close and listen!) I've not got an awful lot more to add. I really just wanted to say that this is one of the better terrains I've seen submitted recently.
You should definitively either reflect everything or nothing, don't take shortcuts, always do your best effort whenever you do a terrain. It's all about spending quality time with your little piece of art.
And I completely agree with Gilles, that sky looks awfully stretched and out of place, using smaller versions of the cloudspheres 1, 2 and 3, makes for much better clouds, you could even experiment with tilting them.
Also, adding stars behind the clouds always make for a nice effect, personally I like using the "Stars" doodad and scale it up outrageously, then spread some of them irregularly behind the clouds, sure it doesn't look very realistic when putting them up there, what with all them stars moving around like crazy, but if done correctly they will make for a very natural looking dotted sky. Their movement make it look less symmetrical, and we all know that a starry sky is nowhere near symmetrical.
I'd also love to see some fog, and another two or even three layers of mountains in the background, you should always use fog and mountains to make the horizon of your terrain transcend and smoothly merge with the sky, in a natural manner.
There's probably more I want to say, but I'm a bit short on time here so that'll be it from me for now. Hopefully, my suggestions might be of some help.
