Oh, this is probably really a spoiler, but I Found Fat Shy Guy's inital taste of the cake hilarious. I didn't see that coming. Also when the Ghost out of nowhere appears and scares that little Toad.
There were also a few little hints at earlier games, like after defeating the Goomba King, he flies into the air and hits the "hill" in the background, leaving a band-aid on it, just like when you defeated Castle Four in SMW.
Kweeh! Forget Whark! Kweeh! Is better!