Obviously F:NV is not set in Washington D.C, and is instead set in "New Vegas". New Vegas itself is still intact, and there seems to be very little in the way of ruins to explore. One of the best things about FO3 was exploring the ruins of D.C. , fighting Supermutants, making your way through ruined buildings, subways ect. Running around in an open desert doesn't seem very interesting. What is there in the game to add a little exploration, and unknown?
I think the exploration is going to be more about factions, people, quests, and whatnot, instead of Fallout 3's which was more about wastes, ruins, and super mutants.