...the atmosphere, the dark ironies of what humans become after a nuclear apocalypse, and poking fun at the 'duck and cover' 50s optimism towards what will happen after a nuclear apocalypse.
Hehehe, remember kids, if there's a sudden bright flash of light and you're instantly incinerated, make your way to your nearest vault and be sure to wear your cold fusion power armor to protect you from the radiation!
EDIT: Oh, wait I'm not done, when it's finally safe to get out of there, don't forget to side with the enclave, the group of genocidal government remnants that are responsible for the original war that caused the widespread mutations they are trying to wipe out, and with their thoughtlessness of consequences more or less doomed humanity to a brutal post-apocalyptic existance.
Or you can side with the super-mutants who are trying to improve humanity with their method of massive mutation that turns people into psychotic monstrosities that with 99% of the population also makes them incredibly dumb.
Or hey, you can side with the pro-human movement of people zealously trying to bring humanity back together under their control with their technological advantage that they are trying to accumulate, they don't actually care much about the people out there, they just want to get stronger then everyone else, and they might actually wind up helping people in the end.
Or you can side with the good guys... Oh wait, there aren't any, doesn't mean that you can't be the only
real good guys though, that will unfortunately mean that you get shot at and attacked by everyone else, and do little actual good in the world, also you will be more or less alone in the entire thing.
Don't forget that your original family and home will betray you in the end and exile you to live on the horribly rough surface world for idiotic reasons, I mean no good deed goes unpunished right? Even if you saved them from death/mutation.