(copying the screenshot here so that i can double-size it

)
<yoda>Impressed by cave story you have been, heeh ?</yoda> Well, for one thing, i'd say your dirt-to-plain-black transition is too fast. You could use some intermediary tiles such as few rocks sparsely showing on plain black, and you could possibly use them in a way so that they don't exactly follow the ground line to make it more interresting to watch.
Regarding the cracks, you may want to have highlight below the crack and shadow above it (a look at the picture below should make my point obvious). And you may want to re-consider your strings, too. Even in mithrill, i'm unsure they could lift such blocks, and they'll be much more interesting to draw if they're large enough to show rustiness and shining areas.

Finally, if you're to revise your background rocks, they should be more showing where the light comes from. I doodled this after my few hours of Doukutsu, and posted
my step-by-step "technique" on my blog :
