“The Lord of the Rings is not, when you boil it down, about Gandalf's magical powers or even Aragorn's orc-slaying skills. It's about the Hobbits, arguably the weakest (physically and magically) of the people in the books. It's about Aragorn's struggle to become king.” This quote from Mr. Sanderson’s aforementioned essay perfectly illustrates Sanderson’s Second Law. Epic fantasy is built around familiarity within “the strange”. Most epic fantasy protagonists are everymen, characters with seemingly ordinary qualities, at least at first. Readers can relate to their weaknesses because they share some of them. Therefore, if a magic system has infinite potential, the same sense of familiarity is shed. On a separate note, having limitless magic results in two possible methods of problem-solving: a deus ex machina occurs (Latin for “god from the machine”, in which the day is saved miraculously at the brink of disaster) or readers are left baffled as something other than a deus ex machina occurs. Both of these are inherently negative.
For a magic system to adhere to Sanderson’s Second Law, one side of the overall equation of the Law must change. Either limitations need to be put in place or powers need to be stripped. In some cases, placing a limit on the magic will involve stripping power. Brandon broke down the term “limitations” into “limitations”, “weaknesses”, and “costs”. The self-named sub-set of “limitations” represents the subtraction of power, while adding “weaknesses” and “costs” represents an addition to the original limitations, in reference to the Law’s equation.
Sanderson summed “weaknesses” up better than I ever could (it is his law, after all). “Weaknesses are things that enemies can exploit—rather than being things the power cannot do, they are things the power is vulnerable to”. The easiest example is kryptonite. If you want Superman out of the way you need only to find some kryptonite. Weaknesses are sometimes generically called “kryptonites”. In this way, you could consider kryptonite an archetype.
Costs are far simpler. The Dungeons and Dragons system of costs is one of the best. For example, in version 3.5, to cast the spell sequester you must have a few “material components”. These are: a basilisk eyelash, gum arabic, and a dram of whitewash. A clever mode of using costs is bringing your characters to a point where they are very close to resolving the conflict, but one last thing stands in their way. It cannot be this simple, of course. In order to overcome the final obstacle, a costly action must be performed. The quest is not over, folks. We need to find some elbow grease and newt eyes to make an acid strong enough to break the chains holding back an ancient demi-god. Costs can therefore be used not only to make a better magic system, they can also create brilliant plot points.
Slashing powers will get a broken magic system out of a rut faster than perhaps anything else. This does not necessarily mean that the powers themselves must be cut, only that the total power of magic in the world is lessened. In my current work-in-progress, magic can only be performed under two conditions. You must have a magic prism and there must be some source of celestial light (sunlight or moonlight). Few prisms exist, keeping magic in check within my system. Also, seven varieties of prisms exist, each with their own power pools. The need for light can be considered both a limitation and a weakness. Magic does not work during the time when neither the sun nor the moon is up, a limit, resulting in the possibility for exploitation, a weakness.