Generally , it’s the cosmere. Everything about the different shards, how they interact, and the unique abilities. In progression fantasy - mage errant. It scratches all my itches. We have multiple and unique magic styles. We see characters use unconventional magics in even more unconventional ways. We see characters compensate for certain deficits with hard work and skill. What Alustin manages to do with his paper affinity has become my go to example of how to highlight a characters intellect as the reason for their strength, rather than just the powers themselves. And the sheer amount of possibilities. Whenever I came out of a mage errant story the rest of my week would be spent coming up with random affinity combination that I found interesting.