bevinator: Having played 1+2+3+NV, I really feel like in many ways, the Fallout games are split into two separate series.
FO3 feels like a direct sequel to 1, with plot elements from 2 mixed in. The feel of the game, the atmosphere, the setting, just really hits the FO1 vibe for me. That's my favorite thing about 3, really. I enjoy it, but it makes me want to play Oblivion instead. FO1 is dark, gritty, and nasty. Nothing is clean and shiny and well-constructed. The closest things to a "clean" place were Adytum, which was a despotic warzone, and the Cathedral, which was only happy and pretty above the surface. FO1 also wasn't trying to be funny. There were a lot of moments of dark humor, but nothing that would really make you laugh out loud (except for the crashed alien ship which gets me every time). There were a few pop culture references, but they were mostly subdued and not glaringly obvious.
For FO2, however, a lot of the design staff had changed from the first game. In many ways, FO2 didn't really "feel" like Fallout to me, even though it LOOKED like Fallout. The main character, a backwater tribal, can easily master computer hacking at early levels despite growing up in a village hundreds of miles away from the nearest source of electricity. Many of the cities were clean, bright, and even had uniformed police! They looked like they were stuck in a pre-war time bubble or something! Civilization was back in full swing now, just with fewer resources. The pop culture references were rampant and jarring, and the game felt like it was trying to keep me amused, rather than appealing to my darker nature. It was an awesome game, but it almost felt like it was set in an alternate universe from the first.
As mentioned above, 3 FEELS more like a sequel to FO1, even if it plays like a totally different game. The quests are simpler and more linear for the most part, the combat and NPC interaction are very different, but the atmosphere is the same. It's after the end, and the Capital Wasteland hasn't really begun to rebuild like the West has done. The whole place is brown, practically everything is busted, everyone hates each other, just like the first game. It didn't really feel like a Fallout GAME, but it definitely felt like you were living in the Fallout world.
To me, NV just feels like FO2++. There's bright shiny cities, powerful armies, organized governments, lots more greenery, and the like. The plot and the quests were more complex than 3, but nowhere near as complex as 1 or 2. It felt (more) like one of the original games, but the setting and the atmosphere were all off. It's like all of the stuff that bothered me about FO2 (compared to FO1) turned up to eleven.
My two cents.
Some things to consider... the time between the games is in terms of generations, not a few weeks or years. So it is only natural that as time went on the people cleaned up and rebuilt, and to make makes the most sense. That's where I think FO3 dropped to ball and failed to make sense at the very beginning... it takes place at least 200 years after the war and not one piece of rubble it seems has been picked up or anything.... that always bothered me about the game. Just look at pictures of present day Hiroshima or Nagasaki... granted we have resources and equipment we can use that isn't in the Fallout universe but still, something would have been done. No?.
Also about FO3 being a sequel to 1 with elements from 2.... to me I see it as a shoddy remake of 1 with elements of 2... I mean think about it. What the main plot of 1? You need to find something so that your people can have basically unlimited fresh water. Then there's this army of mutants.... gotta stop them.... Plot of 3? Stop the army of mutants and give fresh water to everyone. Okay you get some choices to make about those in 3 but still its the same premise. Oh and in 3 there are Enclave people, so there's your tie in with 2. I only realized this recently and it really makes me annoyed with the game more. They basically stole plot from 1 and 2 and threw it together in a shoddy fashion, with the most horribly conceived tutorial/opening segment ever.
But that's just me, feel free to disagree.... you'll be wrong of course but still feel free 0=)