Here are mine:
1)Detailed 50s' culture.
2) Liquid situation, the fate of the factions and ultimately, the entire wasteland depends on the player's choices.
3)Fine -contrary to blunt - differences in quest endings. - The palette of possible outcomes is wide.
4)Moral ambiguity - No more black and white situations please. Fallout 1 had isometric graphics, but multi dimensional characters... With stories, lives.. understandable and human motivations with thoughts of the land. I think this made us stick to those characters, we felt empathy towards them, and could understand their plight.. a little. I loved that, maybe more than everything else in the Game.
5 ) FUN! FUN! FUN! Insider jokes, Easter eggs, outside references; creative and absurd settings, killings etc. The Fallout Universe was just MADE to be the stage of such jokes.
6)Vaults. High tech encl.... "havens", very concentrated, tight packed, tech-intensive societies, the bigger the contrast with the wasteland, the higher the satisfaction coming from bridging the gap.
7) And finally, preserving the Ironic attitude about itself the whole time.