First, you can play through the 50 levels across 8 game worlds. That's pretty huge for an eShop-only game.
Also, there are "hidden advanced ways of playing" - According to the Feb 14 Nintendo Direct, you can play the game as is or you can also be creative and find new ways to play through the game. What's more is that there is Pokemon-themed music in HarmoKnight as well. Wow.