Thanks for your replies!

You're right, I should have picked one thing and asked about how to improve that single thing and not post all my work at once and expect feedback.

You've guessed it already. The stuff I want to fix the most are the tiles/tilesets. Right now, I have a pretty flexible setup which lets me assign a certain texture and colors to each single tile. I have a sort of tile "prototype" which contains all the variations in texture:

Then I have several color-sets, each 3 colors. The black, white and read pixels in the texture get replaced by the colors in the chosen set. I can mix texture and colors as I like but in practice, only a handful really make sense and look ok. I might have to break away from this method completely and create traditional tilesets. I just thought I could be lazy but now I think it really shows...

And yes, I currently don't have any transitions between the different tilesets. You think that's a must?
I kinda fear that 8x8 is too small as a tilesize as I don't even know how to produce tiles which are tileable and still not feel natural and non-repetitive.

I'm just fishing for ideas right now, so please tell me whatever comes to your mind.
