Also... You are the first person on this forum to know who Tool is. huh?
The main problem with the tileset is indeed that it's too bland; nothing has any texture at all. The colors are also poorly chosen - too much contrast in some places, too little in others, and too much saturation overall. Pretty much everything in the bottom left is too scribbly/messy/MSPainty.
Do you mind if I make a remix of this? Hopefully I can do it well...
Very nice tileset by the way. (I'm commenting on this now because I am only just now discovering the Development Showcase )