(* or the allure of the sidequest)
Listening to the Rock Paper Shotgun podcast a couple of weeks ago, Kieron Gillen and Alec Meer raised a really good point about open-world games, specifically about the importance of side quests. In most open world games, the player is charged with a task that they must fulfil, usually involving some kind of imminent threat to the world itself. Now this task is usually the bulk of the main story and should be the players' primary motivation in advancing in the game. The problem is that there are side-quests to do and this is where things get confusing. I know I for one tend to sway off the path intended for me, sometimes leaving the main storyline for a big chunk of my play time.
Deus Ex had the right kind of idea: give the player complete freedom, but define the parameters in which he can play. In Deus Ex, you're presented with a mission, with a clear objective and goal, but you're free to achieve these however you want. This involves either being combat heavy or stealthy; ignore optional objectives or be a completest. But the goal doesn't change, you still have to keep moving the story forwards. The openness of the game is about pace and your approach to the problems in front of you. Both STALKER games tried to accomplish the same thing, but relentless back tracking sullied the games' enjoyment.
So what's the solution? Well, how about a shorter main quest? Get to saving the world quicker and make it the only thing you want to do. Motivate the player to stay on the main quest, but don't do it at the expense of losing any kind of motivation to do the side quests. Maybe lock off a lot of content until the main quest is completed. Or once the main story line is done, open up more quests in a linear fashion, giving the illusion of the main game continuing. Or have the main story take diversions that keep the player occupied and engaged. I'm not sure what can be done without limiting what freedom a player has. However, it wasn't long ago that games were just a successive stream of levels with no way of changing course or exploring.