I'd say the feel of the game. In Fallout 3 it had that "I'm in a nuclear apocalypse wasteland that I must survive in" While New Vegas felt more "I'm in a desert with some big bugs... oh look a giant town." It didn't feel like I was fighting for survival. There was the legion but they were so organized. And there was no reward to exploring anywhere, find a few bandits somwere in the middle of the desert but again nothing that screams NUCLEAR APOCALYPSE!!
What about for you?