Massive Fallout 3 fan here who never played NV, or any of the previous games.
In addition to this, I actually loved the ending to FO3, loved the VATS system, and really, really hope that there is no co-op or online element to FO4 unless it has absolutely zero impact on the single player game.
I'd certainly like to see a greater variety of enemies, crave more of that lovely 1950s music, and hope that I start to hear "heroic" announcements about my character's achievements over the radio.
And, as reported in one of the previous threads (I think) a map that is multiple times the size of Skyrim's? Oh, yes please, mother!