I think Radioactive has it right. I think a lot of people are taking things into account by relating the way things were built in 2077 to how they are built now. considering how advanced things were when the bombs fell I don't think structures during that period surviving 200 years later is a stretch.
I think one thing that is pretty obvious to me is that there was the threat of a nuclear war long before it actually happened since there had to be a reason for the Vaults to be built. So with that in mind all sorts of new technology could of been developed to help make vehicles, houses, and other structures and objects survive a nuclear blast. There is also the possibility that new types of nuclear weapons were developed by 2077 that would have different effects then the ones we have now would have.
As for weapons and ammo, well it has been stated before in past Fallout games that people cast their own bullets. No reason they are not able to cast their own bullets for different guns and what not. Also, considering how hard it might to get into old military bases and such in the area that is another reason those places are untouched. Additionally just because there isn't any sort of education system doesn't mean people are stupid, people are going to horde their ammo, store it up somewhere safe so it will be there when they need it. They aren't going to go and waste it by shooting at nothing. And considering how much this stuff can be stolen, moved around, forgetting during a crisis, ect ect I am not bothered by how common it is to trip over an ammo box in a sewer or find a store room full of ammo.
I just think some of the complains about the game along these lines are sort of silly. Now I will admit that in reality all of these things, guns, ammo, food, drugs, standing buildings, ect would all be very rare 200 years after a nuclear war. There are two things to take into account here. 1) this is a game that sort of needs that stuff for it to play right, so yeah, that is why this stuff is so common when it wouldn't be, the game needs it to work. 2) if the designers took thse ideas into account, outside of the super mutants, BoS, Vault dwellers, and people who got lucky and found intact facilities and such, the majority of humans in the wasteland would more or less be cave men since they would not have anything else to work with.
