Fallout 3's world was a dense destroyed and ruined urban environment in the east, and plenty of burnt and irradiated countryside to roam around in. The whole way through my 100 hour plus Capital Wasteland experience in felt very authentic - It made me feel like I was in an actual post-nuclear discharge wasteland. Sure it would have been nice to have had all NV's factions, but in the end a game is judged on quality - something F3 had lots of.
Now New Vegas. The NV world all the way felt like plain old desert with, sometimes uninspired creatures to encounter (not all the creatures are terrible - mind you. It's also notable that in NV you had to go find action - in F3 it found you), stitched together by a Las Vegas strip, barely a 6th of the size it should be. In Fallout 3 all the ruins and monuments were true to their actual size, and the noticeable lack of walk able sky high ways and F3's huge metro system is another letdown. You cannot just relay on a famous location to provide a good game.
When I spent my hard earned cash on NV, I was expecting a F3 world with a side of the New California Republic and Caesar's Legion. When I first booted up my console played for a few hours expecting the find Megatons and wrecked towns. The F3 post apocalyptic townships have almost disappeared, and wrecked towns are a rarity all over the map, with the exception of Freeside and Westside.
You may be wondering what I mean by 'F3 post apocalyptic townships'. I mean towns struggling to survive against raiders and mutated creatures, built with scrap. (i.e Rivet City and Megaton). In NV it felt like the people who live in the towns just let any old person walk in, only 2 towns comes to mind with some form of fortification: Novac and Westside. I'm not really sure if the walls around Freeside are for defense or just the keep the rif-raf away from the NV strip.
In conclusion: NV really felt like a missed opportunity to create a engaging world like F3s. I was excited by the concept of new Factions and a post apocalyptic Las Vegas, but I was highly disappointed.