Very nice, especially the first two screenshots look amazing!
Only the 4th shot really does not resonate with me, the rock structure and texture work looks subpar and so 2004. It looks like the neat mushroom background art and the foreground art changed places accidentally.
The seam to the ground texture looks displeasing, the rock texture is too repetitious because of its defined bright and dark parts, the structure seems to lack some polygons here and there, there are no real details like stuff on the ground, small mushrooms or other stuff growing on the walls and so on. What you could do is try to create a subtle highlight on the borders because the light is coming from the back, then choose a less distinct texture, apply triplanar multitexturing and multiply it with detail and brightness maps or use some sin(inPos.x/256+inPos.y/32) brightness modification or something like that.

Why don't you create a stand-alone WIP thread in this Projects Forum and keep us updated there?