The Idea is a Distant Cold Planet, with a new Volcano being born out of it, so here are my thoughts in point form.

• Right now it looks very cheezy and amaturish for me.
• Cant seem to get the snow to look as good as what I've seen done with Bryce.
• The distant planet and mountains just dont look right, I'm guessing its a scaling issue.
• Bad attempt at Lava.
• where terrains meet each other look squarish.
• I chose the Default  Star Field night sky, but the stars look too big and squarish too me.
• the ground revealed by the melting snow looks too muddy.

Did I leave anything out?? :)

well if its a lava land, you need vapours, and maybe a very thin atmospher. I understand its space so might not be much atmosphere on your planet, but its still realistic for vulcano's to spew fumes of some kind. this would also let ice melt, so some watervapour would also be there. For the snow, maybe.. try a but of texture rather then just one color. also the shape of the surface does a lot sometimes. maybe longer whispier shapes work better? The muddy ground could do with some more variation and texture to make it look less muddy. for the stars, google greg martin's star tutorial ;-) the mountains dont look distant due to lack of haze. the squareness could be solved by more variation in elevation. lava: check out pidjy's tutorials. they should be somewhere in the forum archives.

Tricky Scene. Frozen Snow & Hot Lava.

The transitions between the 2 can be done with duplicate terrains of different materials slightly seperated. The snow has a lot of ambient light bouncing off it. You might try cutting back the ambient dial or even making it slightly transparent over another material. Also try more texture(bump).

The complexity of transition between materials here is the key, Might even try Spray Render techniques.
Check this out.
http://designbydeanna.net/bryce/spraytutorial.html

This Scene and others by pjero:

http://www.renderosity.com/mod/gallery/index.php?image_id=556173&member

(great seperation here)

Keep us posted........

I like it so far. There is just one thing i'd like to add that you might consider.....
Select your whole scene and group it, and then enlarge the whole scene. IMO the distant haze looks so more realistic. experiment with the size and do quick renders to see the result.
Also a little more glow from the lava. Raise the opacity of the mat and place a radial light source just underneath the surface. You may have to experiment a little getting the opacity right. Of course this would work better with volumetric world turned on, but will significantly slow down your renders....even in quick mode LOL, but it would be worth it. Good luck! :-)