I'll definitely be doing some wooden structure (think Beach) and beams that'll allow decoration and variation in the mine. The screenshots you provided Stijn, are definitely very useful and I'll be adding both the lamp as well as sacks with gems/gold for decoration.

I'm thinking of doing minecarts the way I did the carts in my rollercoaster set, they're either full of something (gems/gold) or empty with some event you can place in them.

I also did the backlit effect on the torches and it came out looking much more dimensional - but I won't be doing flickering light animations, I'm afraid my method of doing it would eat way too much color. However, this is how I made the effect:



I think it added a lot of depth to the area, so it was definitely a good suggestion. I also changed the tone of the rock to brown.
I choose circles for artistic reasons, actually. It's easier to split shapes stacked together (like the leaves in my rollercoaster tileset). Circles are also easier to form and shade, so it's just within the style.