I don’t get it either, I liked the main plot in DA:O but most of the game is dealing with some big side quests that have next to nothing to do with it.
I don’t remember having many big decisions either, a few characters might die but it doesn’t really matter in the end.
Fallout 3 takes place 200 years after the war and, like I said, the main story is terrible but most of the game has nothing to do with it.
New Vegas’ world is mostly empty and static, the writing and RPG mechanics like reputation are what make the game as good as it is.
The thing about New Vegas vs Fallout 3 is that they have different target audiences, Fallout 3 is a game about exploring and immersion while New Vegas is an old school rpg with big decisions central hubs.
The capitol wasteland is in much worse shape than the Mohave. From what we see in Fallout 3, people generally eat squirrels, iguanas and pre-war food.
The capitol wasteland pretty much works on the basis of scavenging and trading with the occasional herd of brahmin or small farm here and there.
Fallout 3 also has a big emphasis on water, Megaton has a water purifier that is about to give out while other settlements just drink irradiated water or trade with merchants.
The main story in Fallout 3 is terrible but the world building is pretty solid overall.