In FO3 I often felt I was lost and on my own out in the wastes or within a vast complex like the War Memorial or the sewers that led to Our Lady of Hope which in turn led to the hospital and then to a hotel (vast and huge all together).
However in FO3, I felt the factions were anemic with no quests, no background conversations and history - all things that FO:NV has in spades.
Yet, FO:NV has almost no vast places - except for the vaults. The world feels streamlined and small, pockets of world put in place for the story that takes place there - all crammed together.
I lust and drool for the large world/dungeons of FO3 combined with the faction quests/interaction of FO:NV......
Just my .02.
J