One of the editors over at Resolution Magazine has posted an interesting write-up about how Fallout 3's narrative and setting were the driving factors for him to relentlessly play the game through to its ending.
The narrative is fleshed out through the player experiencing the environment, with Fallout 3's world providing more stories to the player than its fractured bands of inhabitants. With every generic quest comes the opportunity to explore: an abandoned house shows two skeletons hugging, a piece of lingerie is hidden in an underground metro station, and an innocent looking abandoned village turns into a death trap. There are countless more tiny atmosphere-dredged experiences dotted across the game's take on post-apocalyptic Washington DC. Preening through these areas is always far outside of the boundaries of the main quest, and with Bethesda delivering narrative through exploration it's these broader events which propel players forward with far more gusto than the game's ultimate goal of restoring or sabotaging your father's life work.
The emphasis on exploration is strengthened by the relative ignorance of the player. The catastrophic events leading to the world's apocalypse are rarely expressed in words, quests or actions, and can often only be partially gleamed by scrubbing the environment for occasional clues - although Bethesda do pull on some of these threads for the meat of their DLC expansions. This is also why the initial hours of Fallout 3 are so daunting, with the game throwing you into its stylised environment with far too many weapons, traits, perks, stats and items without explaining how to use any of them.