Imagine a dungeon created by a player from the ground up. Imagine a team of 'real' players entering the dungeon and playing through the adventure while the dungeon's creator (the dungeon master) follows their every move, changing things within the game world according to what the players do in the game environment. Imagine the enormous potential to totally screw them up at every turn (if you're that way inclined) or to help them along if they get stuck in a certain part of the dungeon. You may think creating a detailed environment with a realistic landscape, puzzles and challenging monsters would take an absolute age and a degree in programming.