Anyways, I like NV better myself, but I'm getting tired of hearing about them being completely different or something.
They're thematically quite different, even if mechanically quite similar. KotOR 1 and KotOR 2 is also a good example of this.
Of course Obsidian were happy to work on Fallout, Avellone and Sawyer are pretty much Fallout fanboys. But they are different games in the sense that their design and narrative philosophies differed. One way I'd put it is that Bethesda focuses on environmental exploration, using characters and quests as a means of taking the player from setpiece to setpiece. It's like a themepark. Whereas Obsidian focused on factions and ideologies first and foremost, using characters and locales to drive home the themes and narrative tones they wanted to establish.
Fallout as a series is based on the latter, not the former. The former method is a big money maker for Bethesda (just look at Skyrim) so it's not like I expect them to try the latter (which makes the noted changes for F4 even more confusing) but simply put, Bethesda doesn't understand the Fallout franchise. That's not to say they'll make a bad game, it's just that Fallout is a series fundamentally about society whereas Bethesda have shown that they think Fallout is about the player.
One of these days, I will channel my autism and actually explain "Bethesda doesn't get Fallout" in un-necessary detail and just link back to it from that point. But until I have the time and inclination to write a digital monument to my pathetic grognard inner nature, you'll just have to take my word for it.