It looks fairly flat (like the Banbo fence area's look like they could be laying on the floor, the colour need graduating at the bottom being darker and getting light the higher up, and a shadow could also help make them look 3D)... The way you made the wooden platform with the shadow behind it is what I mean, that looks wicked...
I know your trying to make some thing very custom and not to the RTP style, and that can be hard...
If you was to make this more RTP standard you need to make some shadder and altenative ground tiles, may be even some auto tiles to go with it...
Saying that though I think it's a good start to some thing that could turn out to be very interesting.
I know your first strength isn't graphics, but what I am seeing is very good and I hope you keep at it.