In opposition to the other answers- uniqueness and creativity.

Fireballs and crystal balls are a dime a dozen. Spells like summoning rains of knifes and seeing through mirrors are better. Creating flocks of birds inside people or seeing through the darkness someone makes when blinking is best.

There are a lot of magic systems out there, and honestly there's only so many magic effects on paper (kill people, control people, heal people, move places..). The ones that capture my imagination are those that have particularly unique "fluff", to steal an rpg term. I'll take an interesting magic system that runs on incoherent dream-logic over a consistent one that's another four-elements-and-spellbooks with minor tweaks any day.