Let me start by saying that I couldn't care less about the definitional aspect of this discussion. I don't need a fine delineation of what defines an RPG. Those arguments are seldom interesting and never productive (ever had the "What is Art?" discussion?). What I am interested in is what the function of the GM is and who drives the story forward. Several years ago, if you had asked me why I played these kinds of games I would have answered these questions with the same answer: the role of the GM is to tell a story and it is that person who drives the narrative. I don't believe that anymore.
My friend who rotates GM duties in our weekly DCC group and I were actually having this discussion pretty recently. He likes GMing because he likes to tell a story. He was curious why I enjoyed the role. A big part of that is simply because I've been a GM since day one of my gaming career. Immediately after finishing running my first adventure in which I, an Elven Rogue, broke into a tower to steal a precious gem for the local thieves guild, my friend handed me the Dungeon Master's Guide and Monster Manual and said, "Your turn." Now, I enjoy GMing because I enjoy challenging players.
That is the role of the GM: to challenge the players. Be it a tough monster, a dungeon littered with traps, an overgrown forest, or a moral dilemma, it is the GM's job to place obstacles in the player's path. However, these things cannot exist in a vacuum. That tough monster, was he the final baddie of the campaign or something smaller? Who set all these traps and what were they trying to hide? Why would anyone choose to go through the overgrown forest? If you choose to kill this child you are murderers, but if you let him live the great evil that he has been cursed with will awaken. What do you do?
The second part of this will go up Friday, addressing who drives the story.