Fallout 3 is a game that is so big and open, that most of the time it cannot maintain its substance and style.
The grim post-apocalyptic world and the bizarre underground life presented during the first couple of hours of gameplay, where every choice seems to unveil more of the plot or change it to another direction, creates a very immersive experience. However, not long after you leave your underground life, the game becomes so big and loose that the core essence of the game dissipates and every choice you make is less relevant to the outcome of the game.
Fallout's 3 quests are trite, corny and sometimes simply boring and/or uninteresting. Having a whole world of monotonous errands, accompanied with slow and repetitive conversations with void characters doesn't really compel you to accept quests, which is what gives the game it's life length.
However, as an RPG, what really keeps you on playing is the compulsory urge to level up and improve your abilities and for that you need to kill things. That being said, the combat system in Fallout 3 is really fun thanks to the VATS system, the weapon repair system, the restriction of items to carry around, the health system which can get you sick with radiation or addicted to pills and other RPG mechanics, makes survival engaging.
But, since trying to avoid quests, that most of them take place inside dungeons (inside buildings, sewer systems, mines, etc) and not the wasteland, leaves only the quests that pertain to the core of the story, which renders the game very small and linear.
Big and open yet loose and trite or tight and small, but linear and short. It is up to you to follow the corny and weak side stories or adhere to the more concise and engaging central plot if you don't mind leaving the majority of the wasteland unexplored.
Was this fault of the writers and programmers or is it simply impossible to sustain the essence in a game this big?