The song. It will get drilled into your head deeper and deeper the longer it takes to find that Last Lousy Sphere.
The track that plays on the "No Way!" screen is pretty catchy.
That One Level: Certain block combinations are insane. For example, Block 120note All the blocks in level 46324857. has a dead end, seemingly impossible to go back. It might increase difficulty if the block only shows up once, but if there are more of them, good luck.