The best thing about Fallout New Vegas was right there in Bethesda's initial pitch to Obsidian

The thing that makes Fallout New Vegas endlessly replayable is being able to choose who to side with out of its various factions. You want to work for the NCR? You want to help the Followers of the Apocalypse? You want to collect scalps for Caesar? That's on you.

Looking back at Fallout 3 for Game Informer's oral history, designer Emil Pagliarulo pointed out that in Bethesda's first Fallout, "There are no factions. You can't join the factions, right? You join the Brotherhood of Steel, but that's the main quest.