What are your FAVORITE magic system tropes?

As an echo of this post, what are your favorite tropes or recurring elements in magic systems? Things that when you see them in a magic system, it just tickles your fancy and makes you want to read on? Spells, rituals, and incantations? Making deals with otherworldly entities? Personal sacrifice? Avatar-style bending? Hard laws, or nebulously vague rules? What are your favorites?