Something in the Zak S. post about D&D as a creative outlet made me reflect briefly on the perennial question "Why is there so much medieval European fantasy and not enough other cultural influences?" The quick answer is that it's not all strictly "medieval" or even "European". It's fairy tale medieval. It's a culture we've all been exposed to in Western Society, and isn't even strictly all European, since several fairy tales from Middle Eastern and Indian sources have been included in "European" fairy tale compilations for, oh, a hundred years? So it's stuff everyone can relate to, even if they each prefer something else.
Every GM, even those who hated improvising, is going to have to improvise at some point, or explain something they've prepped. Fairy tale culture is so common and so simple, it's easy to toss out there and just move on with the game.
Society is divided into royals, nobles, and commoners. Tech is pre-industrial. Wizards, witches and fairies can cast spells. All the standard monsters in fairy tales exist. That's am easy-to-grasp background. Anything else requires specific knowledge and negotiation with the players.