As far as outside the walls of the Strip...I don't buy the "it IS a desert" comment that people use to explain New Vegas' lack of...well lack of everything. First off it's a game. Games are supposed to be fun, not boring and empty. Second there are lots of unrealistic things in Fallout games, so the excuse of having the desert deserted like a real desert is not a good one.
I agree with that. The whole scenario is completely ludicrous, so 'realism' doesn't really enter into it. Mutants, ghouls? Get exposed to high levels of radiation and you die slowly and horribly from massive organ failure. You don't get a skin complaint and a husky voice!
I've seen a lot of complaints about Fallout 3, that the characters weren't fleshed out, and the quests were uninteresting. I've been replaying it, because I got bored with NV, and there's a lot of interesting characters. I was talking to Carol in Underworld and her description of seeing her father vaporised in the Great War was actually quite moving. She tells the story reluctantly, which is believable. Everyone in NV wants to tell you their life stories as soon as you meet, which is unbelievable. The only realistic character is the NCR ambassador's secretary, who tells you to get lost!
As well as interesting characters Fallout 3 has interesting locations. I took a wrong turn in the DC metro and ended up in a district I hadn't seen before. I actually had a feeling of being lost, which I never had in NV. On top of being lost in a strange part of town, I found that it was populated with angry raiders, who were TOUGH for my low level character. NV is very easy in comparison. There's no real threats throughout the game if you're reasonably careful.
For me Fallout 3 is just a more fleshed-out, all-round gaming experience. NV is a bit... slight. Good for a couple of characters, then the reams of dialogue start to get a bit tedious. Lots of A to B and back to A negotiation quests. There's little action or exploration to add sauce.