To make it fun, magic has to be hard. If all that is required is to mumble a few words, or wave a magic wand, then magic is cheap. And if magic is cheap, then it loses much of its, er, magic. In Forest of Lost Secrets, magic isn’t something that is inside us. It is out there in the world around us. It can be harvested, but only if you know how, and only if you put in long hours of hard tedious work. One vial of mirclair took Thallach two hundred years to make, toiling away in his workshop. His only break was to traipse around the countryside looking for gold, which was a necessary ingredient.
Even then, even when you have a magical elixir in your hand, you have to know how to use it. Just as in science, where there are laws of nature, there must be laws of magic. These laws can be very different than the laws of science, but the application of magic must follow them. In Forest of Lost Secrets, a drop of elixir must strike the correct run. Then the magic happens. We are told (by Keeva) that it doesn’t matter if the rune or elixir get washed away after that point. Because the spell has taken place at the moment of contact. Other rules are in play. If one transports from one realm to another, it matters where one starts out. Locations in one realm correspond to locations in another. While invisible from the green elixir, one can strike one’s enemy. But while invisible from false mirclair, one cannot. The laws of magic are different.
If there are rules of magic, then the characters in the story know what is needed to use it or acquire it. Jessica could not go to Derfaria if she did not know the correct rune to draw on her arm. Iragram learned how to fudge the laws a little bit when he made false mirclair out of silver, but there were consequences to his fudging. The false elixir didn’t work as well. When the Kyne brothers were transformed to trees, the luth took a while to take hold (since they managed to walk to the enchanted forest before transforming), and even then, the transformation didn’t lock in securely, since the boys, being able to occasionally talk and move, did not completely lose their human characteristics.
The idea of having magic in a story is fun because it allows for things to happen that never happen in real life. But if the magic is cheap, if it is everywhere and spells are easy to come by, then it’s just not special. It becomes almost arbitrary and, I believe, less engaging for the reader. We can compare magical systems to the personalities of the characters. If magic follows rules, the reader becomes familiar with these rules as they read the book and it gives them a deeper connection to the story. And they share in the feeling of accomplishment when the story’s characters make the magic happen. That, I believe, is what makes a story about magic magical.