I'm working on a magic system, and I'm focusing on concept rather than mechanics right now, so it leaves me free to look in various directions for mechanics. I want to do something with a lot of narrative/descriptive play in it, where the effects of spells aren't prescribed but rather "interpreted" or quasi-improvised within the context of the fiction.
What are some of your favorite approaches to magic in games? Why?
ETA: This thread was previously called "Let's talk about magic."