Pipe losing volume when it goes vertical.
http://www.itchstudios.com/psg/art_tut.htm^ Take a look at "Light Stuff" section. Particularly note specular points.
AMB: light from sky (reduced)
REFL: light bouncing off wall up onto pipe
OR: 2 ways I tried here, one with light bouncing up from pipe below, and one without.
Rocks look cartoony/flat due homogeneous rendering. That might be what you want, but the way you've shaded the rocks still doesn't really give attention to the form, which makes it look off. As for cave bg, consider what sort of light levels you want in there, what the contents of the cave would be (are there structural supports? stalactites/stalagmites? open cavities? mine carts? etc), once you know what will be there and how to light it, it should come to you.
Atm the grass looks kind of stylish to me, like a cel-shaded thing going on. I think it would work with extra little bitties above that tile showing tufts sticking up. Got to remember grass grows
up, after all
EDIT: Oh and the giant cog/gear sticking out the side of building; remember the function of a cog, it has to interlock with other cogs. Atm you have uneven teeth on it, which in my mind makes it pretty pointless, and really makes it out of place in the image.
