I'm impressed by the balance of the rune challenges in this game. They're timed to perfection, often demanding near-perfect execution and coming down to the last second. And by the time you complete them (often after multiple attempts), you feel like you've truly mastered the skill that will be improved by the rune being awarded.
For example, one challenge gives you a rune that increases your air control after a jump. The challenge for it has you jumping with precision from one time extension to the other, and it comes down to the last second. Another challenge starts you off with critical health and gives you a limited amount of time to take a variety of enemies, forcing you to rely on armor pickups. As a fitting reward, you get a rune that makes enemies drop armor. It came down to me and a Pinky, with less than 30 seconds on the clock and I only had 1HP and 2 armor points!
They could make a whole WarioWare-style spinoff of these rune challenges and it would be a brilliant game in its own right.