Something I should have said in my original reply... to understand an Oblivion or Fable 2 you have to try and avoid thinking along the lines of "main" and "side" quests. Oblivion is structured so that you have several quest lines (fighter's guild, mage's guild, thieve's guild, assassin's guild), along with a "main" questline that adds a "main story" to the open exploration, along with a lot of side quests that you find simply by wandering around, such as the vampire and artifact questlines in the imperial city, avenging a farmer's late wife, etc. And there are a lot of dungeons that aren't even a part of a quest line... they just exist for the exploration and treasure hunting.
I'm hoping Fable 2 is more of the same with a lot of refinements.
