Magic must have rules, laws and imperfections.
1) Without rules, it’d either be incredibly frustrating to read the result or extremely boring for the reader. To quote Al Murray, “if we had no rules where would we be? France!”
2) Without a governing law, magic could just spiral out of control with no comparison point. However, the thought of magic becoming so wild and uncontrolled that it breaks through the governing laws adds an element to a story which gets you several bonus points.
3) Imperfection, the ability to fail, not only makes a story more realistic (I mean in terms of narrative structure, not literally – there’s magic involved!) but can provide either an advantage or a disadvantage to your protagonists.
Magic can appear in several ways. There’s last week’s mention of being able to be controlled by people, and used as a tool (or worse). This carries implications that something might go wrong with their control. An alternative is that magic pre-exists in the universe (somewhere, a Star Wars fan is hissing “Midichlorians” – an exaggerated but appropriate example) and tapping into it is either done by the magic-wielders from earlier or leaks out and causes chaos in nature by itself. The third, surprisingly common form of magic is through a magic object or artefact that has a significant effect on the world around it.
The point I’m trying to make is that all three: people, pre-existence and objects are somewhat dependent on each other, like some kind of holy trinity.