I can totally see your point when it comes to quest design, but in terms of world-building I have to say I feet the absolute opposite; in NV I'm almost literally railroaded by the map design, which is essentially a big highway with unexplorable canyons/empty, mob-filled desert on either side. I have done so many loops on the highway now it's getting really boring, and I find myself using quick-travel far more than I ever did in 3. It wouldn't be so bad but I know I'm not going to stumble across any random encounters etc. when walking the same stratch of highway for the fiftieth time, just the occasional travelling merchant or angry fiend. Basically, I really dislike the feeling of having to stick to the roads.
On the question of DC feeling like a real place, what I loved was the picture I got of life just before/as the bombs began to fall. New Vegas may have more life in it currently, but to me it doesn't look like it was ever a pre-war city. Sure, there's the Strip, but that's just one street essentially and aside from that Vegas might as well be A.N. Other small town. I mean, irl it's a city of hundreds of thousands if not millions of people, right? In New Vegas, it doesn't feel like it was ever anything other than a town of a few hundred people. It feels like a real place *now*, but it doesn't feel like it was once upon a time one of the most popular tourist destinations in the world. Downtown DC, on the other hand, is the perfect picture of a huge, sprawling city utterly eviscerated by the war. All that hulking, empty concrete. I find wandering its streets to be incredibly poignant. I love the emptiness of it, just me and the remnants of man's own folly. I really do think FO3 is a remarkable game as a result, but appreciate that this may not be a view shared by those for whom number of quests etc. is the criteria by which a game is judged. Like I say, for me it's all about the atmosphere.
edit: Kjarista sums up what I'm trying to say: