What really makes it is the fact that it's all tilesets custom-made for the pack, which allows the authour greater creative control over the design of the levels.

I agree with what unhit said though. It's a great pack, but does feel like its missing a bit. Personally I think the #1 thing that is really missed is a plotline of any sort. It would have been nice to have a story to bring everything together and give the player some motivation. Even if the unfinished stuff was replaced with text cutscenes between the levels that would have been better.