Why should the player give a shit?
With game stories, always start from the beginning - the player. You are constructing a power fantasy, and the game story should be player-centric and player-reactive: EVERYTHING should be built around the player. We may have carried that philosophy a little too far in Planescape: Torment, but I think the importance of story lies in just one question: "Why should the player give a shit?" If you put this question to every event, every location, every NPC and companion ally, and ANSWER it, then you have the makings of a good story - and even better, a good motivation. The rest is execution.