I wasn't aware there was something preventing exploring in New Vegas.
They're called Cazadores.
I jest, but only partly.
Anyway, Lt. Andronicus pretty much hits the nail on the head for me (good link too, ta!). Stumbling across the Groknak The Barbarian text adventure in the Hubris Comics building, or finding a Raider drug lab in a SAT-COM relay tower; finding a note with "LOOK IN THE GIBSON HOUSE" written on it, or reading a nurse's account of the days following the climix of the war... it was the little details that made exploring so rewarding in FO3, and it NV suffers by comparison in this regard IMO. The number of tiny interior maps doesn't help matters either, where are Vegas' schools, hospitals, office blocks for us to explore?
As to the copy/pasted argument about how much sense all these intact-after-200-years computers etc. make... well, it's a fair point, but me, I'd rather not look a gift horse in the mouth. I'd rather the hint of unrealism if it lets me read all these cool/interesting/funny/sad log entries etc. It's a duller world without them. Again, IMO, lest anyone get their knickers in a twist.
(We've already had the "theme park" argument wheeled out, and even map nodes have been brought up apropos of absolutely nothing... great thread for FO forum bingo, this. Just waiting on "why must the game end" and someone quoting "I'm looking for my father. Middle-aged guy, maybe you've seen him?", then I'll have a full house.)