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. I remember at one point, our lead animator at the time, Hugh Riley, he made a comment in a meeting that said, 'We have the opposite of feature creep. We have feature seep,' meaning that we were cutting things. We were really smart about cutting things, because we knew that we couldn't do it."

Knowing they were heading into Skyrim development shortly after Fallout 3, and that it would be a good long time before they could make another Fallout, Bethesda turned to Obsidian to help fill the gap. As game director Todd Howard said, "We went to [Obsidian Entertainment] and said, 'Hey, would you like to do something?' And all we gave them was, like, 'We want you to do something and use factions.' We didn't do a lot of faction gameplay [in Fallout 3]."

Given a directive to make something Bethesda couldn't do central to their game, Obsidian knocked it out of the park. While the big choice in New Vegas is whether you side with the NCR, Caesar's Legion, Mr. House, or go fully independent, along the way you're making a load of smaller choices about who to side with, and who to sidequest for.

You meet the Boomers as part of the main storyline, but whether you hang around and help them with their giant ant problem and array of busted solar panels is up to you. Same goes for the Kings and the Great Khans and the Three Families and everyone else. Every town has its own measure of your reputation. By the end of the game, the Mojave is a map of people who are thriving or rotting—all because of you.