Hi, good start!
I especially like the foreground and clouds, the composition with foreground, middle- and background is also good.
You asked for comments, so here is my humble opinion:
Your water could need some improvement in my opinion, try changing the wave size, transparency or reflectivity, wind patches could also add to the dynamic and realism of the surface.
If you used a strata shader for your canyon walls you could try to tilt the strata a bit. Maybe change the suns direction for some more dramatic light and shadows.
You can have a starfield shader even today, I built an early prototype long ago, you can see the result in my recent image "ringed".
But there are much better starfields by now, for example dandelOs:
http://www.planetside.co.uk/forums/index.php?topic=8995.msg96122#msg96122Or a bit more sci-fi from pachalmars:
http://forums.planetside.co.uk/index.php?topic=2017.0The great thing about TG is, its team-art, you can use other peoples ideas without stealing them and still make a whole new image of your own.
Have fun!
best regards,
Jan