Wizards

I'm a fan of the Forgotten Realms/D&D type of magic users where the long-lived races like elves see humans as reckless, bordering on insane, for how quickly they race through learning and understanding magic. An elf might take a decade to fully understand a single spell before she wants to use it regularly, a human will race through ALL her learning in 3 years.

And because humans are humans, they just might spend decades pushing a specific magic type or spell to its absolute limits, in a way that elves would never even think of because it's just outright insane.