For Felix, it's the... interactivity, perhaps, of the magic system. You have spells that specifically affect the world environment, not just enemies' hitpoints. And that's something you don't see many other places, certainly not to the same extent.

Fluttershy's canon is friendly, and fun, and smart, and adorable. (Twilight Sparkle is also all of these, but she's not specifically my favorite part of the canon.)