For those of us with a proclivity for the Great Clomping Foot of Nerdism, but who, in our moments of clarity recognize that over-world-building is a barrier between the GM and the players, a tenet:
Don't GM like Tolkein. GM like Chandler.
Raymond Chandler famously resisted plotting out his books, figuring that the only way to keep it surprising was if he himself was surprised. Instead, he concentrated on writing one 800-word segment at a time, and making sure that segment had something juicy in it.
If you gave me the best plot in the world all worked out I could not write it. It would be dead for me.
Of course, if you look back over Chandlers plots, they don't always make sense. But that doesn't matter much, because they were still fun to read.
The ideal mystery was one you would read if the end was missing.