I agree with Psychosniper in that it would be less annoying to have the whole map visible from the get go, but I have to say that it wouldn't be as realistic for a RPG.
In FOT, you were given intel, and a map, of each mission when you started, so of course the whole area would be able to be seen without having to actually explore it. In a RPG, you should have to explore a town that you haven't been to yet to see where everything is. I agree with Shadowbird that the LOS should have a REALLY long range (Pretty much infinite for the purposes of a map that's only one kilometer on a side), and when you get to a town that you haven't been to yet you should be able to pick out the location of every building that you can see (Which is basically the whole map). But if there were to be a small building behind a big one that you would not be able to see in real life, then you should have to actually walk around the big building to be able to see the small building behind it. Afterward, when you come into this town again, the whole map should be revealed because you remember where everything was. The same should be true with a town that you haven't been to yet, but that you have a map for.
The FOT LOS system was excellent in my opinion, in part because of the grayscaling of entities that you *knew* were there, but you could not longer see. With buildings, as soon as you explore a town and then later can no longer see a certain building, it should not be grayed out because you can probably safely assume that the building isn't going anywhere and it should have full colors.
So I think that the FOT LOS system should be in FO3.
Shadowbird said:
2) Not an all-around LOS. No one can see in all directions at once. And this would make party formations meaningful outside of battle, and also allow stuff like enemies following you but hiding when you turn around.
While I agree this would be more realistic, I think it would be too annoying to be worth putting in the game.
Shadowbird also said:
About the sniper characters - LOS should be dependant on perception and - occasionally - on equipment. I.e. - if you have a high PE, you can see and shoot stuff at a further distance. If you equip a sniper rifle, you get a boost to LOS range, but loose it's width (since you're looking through a scope).
If #2 on your list was to be implemented, then I totally agree that this should be part of it.