Weaknesses, limits and costs are more interesting than powers

--- Brandon Sanderson, #2 of Sanderson's Laws of Magic (Wikipedia).

From Wikipedia (references removed):

Sanderson's three laws of magic are creative writing guidelines that can be used to create magic systems for fantasy stories:

1. An author's ability to solve conflict with magic is directly proportional to how well the reader understands said magic.
2. Weaknesses, limits and costs are more interesting than powers.
3. The author should expand on what is already a part of the magic system before something entirely new is added, as this may otherwise entirely change how the magic system fits into the fictional world.

Additionally, there is a zeroth law:

0. Always err on the side of what's awesome.