Quis custodiet ipsos custodes? -Juvenal, Satire VI, lines 347–8

My Rules for Storytelling

1. As Eliezer Yudkowsky put it so eloquently, "You can't make Frodo a Jedi unless you give Sauron the Death Star." Characters that can walk all over any challenges that face them are no fun in RPGs, and they're no fun in a story either. Power balance is key! In fanfiction that's especially important, since some of these characters are already insanely powerful. So they need some sort of handicap, and their opponents need to be just as powerful. Which leads to:

2. If every conflict is the equivalent of a nuclear explosion, you're doing it wrong. Either your characters are too powerful, or their opponents are, or both. Little conflicts can be just as interesting to read as gigantic ones, but when every conflict almost ends the world, it starts to be boring after a while. Basically, don't threaten the end of the world as we know it unless there's a chance it'll actually happen. Mr. Necromancer-Lich-King-of-Doom can pose a terrifying threat without a big red button that unleashes the zombie apocalypse and destroys the planet.

3. Follow the laws of physics! Now, these don't have to be the ones you learn about in college physics classes, but they do need to be consistent. I tend to follow Conservation of Momentum and Conservation of Energy, though in magical worlds there are different sources of energy around to power the magic. Any spell that produces large physical changes requires an equally large amount of the caster's energy. Admittedly, I'm a physics major, so the laws of physics are near and dear to my heart, but every good world I've read about has consistent physical laws.