cos invisible walls totally improve the game right?
It does make tonnes of improvements, just not in the world building department.
I don't know about world building. Sure invisible walls were annoying (but easily fixed by mods, such as http://newvegas.nexusmods.com/mods/45104 which also deals with lots of bugs. I haven't run into a single issue in 20 hours of play), but the Mojave Desert itself was set up far more realistically than the Capital Wasteland. All sorts of organizations have been set up, there are proper trading routes with stops along the route and an actual base of operations for it, there's plenty of agriculture, the sources of electricity are shown (the dam, the HELIOS power plant)... The setting made sense.
In the Capital Wasteland it looked like it had been just a few years since the bombs fell. Megaton, one of the biggest 'cities' around, relies completely on trade since the town basically has 1 brahmin for agriculture. There's usually only one guy with a brahmin standing outside the gates for trading, yet there's a large number of people living in the town, most doing apparently nothing. And all the abandoned buildings/shelters filled with pre-war food and medical supplies that somehow were never looted by anyone in 200 years, until some kid comes out of a vault and decides to look around.
Don't get me wrong I love Fallout 3, but you have to stop yourself from thinking about some stuff too much or it doesn't make much sense. An underground cave filled with kids, with a crappy barrier being the only thing that separates the kids from a vault filled with super mutants that's linked directly to their home? Large amounts of 200 year old food all over the place, that's perfectly edible too?
Fallout 3 may be more enjoyable to explore as a sandbox world, but in my opinion New Vegas has a more believable world.