Being able to actually do anything you understand well enough, the way that magic works in many settings. A brilliant engineer can design an airplane, but he still needs lots of workers and machines and materials to actually build one. A wizard who knows how to cast "fly" can just go ahead and cast it and fly around. If he has the right books, he doesn't need other people for anything at all.

The sort of mystery only possible in a story where the author himself doesn't have to know what the secret is. I love science but I was very disappointed when I was a kid and I realized that science meant that everything ultimately had a mundane explanation. A mysterious structure? Built by some Bronze Age dudes. Dinosaurs? Unusually large animals. Legendary heroes? Made-up stories. Even if elves or unicorns turned out to be real, they would have a mundane explanation too because the whole universe follows rules.