Questions to ask when writing the magic system

This list comes in part from Holly Black's talk on creating a magic system, though I've added to it after a couple of years of thinking about it on my own.

1. How is it learned and executed?

2. How is it accessed?

3. Does it have a will of its own?

4. Is it restricted in space and time?

5. What does available magic do?

6. How does it relate to the character, plot and theme of the book?

7. What is the cost of magic?

8. What can it not do?

9. How long does it last?

10. Who can use it?

11. How do others react to it?

12. Why haven't people with this power taken over the world?