Sanderson's Laws of Magic:
- The author's ability to resolve conflicts in a satisfying way with magic is directly proportional to how the reader understands said magic.
- Weaknesses are more interesting than powers.
- If you change one thing, you change the world.
Basically, the first one says "Don't pull things out of the air. If you want the magic to work, make it REAL and reliable. If you would rather have an air of mystery, which is fine, don't explain the magic—but don't make it do heavy lifting in the plot, either."
The second one says that what the magic CAN'T do is where your story and your character conflict comes from. Allomancy is interesting in part because it relies on metals that can run out. Steelpushing is interesting because you can only Push directly away from yourself.
This forces the characters to work harder, and makes the story more interesting. The most interesting things about Superman or Batman are their flaws—the things they can't do, the things that weaken them, their limitations.
Third, Magic in a world should be interconnected with the politics, economy, science, religion, and everything else. The author must think through the ramifications of changing small things.