any tileset that has less than 20 tiles can't be used for much of a level. one example being Iron crown.

The jj2 tilesets that mortalspaz listed are a bit underused, but it's largely due to the fact, imo, that those tilesets were limiting, somewhat incomplete and/or plain bad, Colony and Beach being the biggest examples

CelL's jj1 letni tileset is another classic example of a badly-made tileset being overused by JCS newbies.