Q: What are the two most important concepts in the D&D game?
A: The most important concept in the game is player choice. In order to give players the most fun in the game, they must be able to make choices that will make a definite difference in the fates of their characters.
The second most important concept is that actions have consequences. Player decisions will lead to further campaign developments.
--Jon Pickens et al., "Dispel Confusion" column, Polyhedron #13
A referee writing an adventure might do well to start by designing the central choice the players must make and its stakes.