Brandon, I believe that if we implemented what you are suggesting, then the game would no longer be an adventure game but a RPG. At some fundamental level, I believe for it to qualify as an adventure, we need to give each player a unique experience that only he/she has access to. Massively multiplayer comes into it as we allow many players to have these unique experiences (the main quests I suggested) within a shared environment/world.
We seem to be approaching the same problem (that of Massively Multiplayer Adventure Games) from two different angles. Obviously mine is more restrictive, but that’s the way I see adventure games. You shouldn’t be leading the player by the nose, but at the same time consider that we’ve never seen a truly non-linear adventure game, probably because it cannot exist.
Anyway, to sum up, I suggested the Main Quests as separate experiences unique to each player (and driven by human or computer controlled NPCs), and added the smaller Side or Sub Quests as ways of allowing players to mix socially with each other and to allow the creation of shared experiences (thus providing the multiplayer element).