So..basically your argument of FO3 being 'broken'is because it's too easy for you.
Bare in mind that players have varying degree of skills.Not everybody is a hardcore RPG-ers that can breeze through games-for example in FO3, reached the tranquility point just after 10-13 hours.
And isn't any game designed to allow any player to be able to survive? otherwise what's the point? There's a reason why you can adjust the difficulty setting.
As far as why DC still looks like that after 200 years...look at what kind of people that still survive after the bombs fell,they're not exactly the kind of people that can rebuild the world...not to mention depleted resources-natural or otherwise, problems with the mutants etc. The US was bombed back to the stone age, and you can't rebuild society in such a short time.
The problem with purifying the water was doing it in a large scale, which was the problem.various characters during the main quest stated this.
As far as characters writing goes...i agree somewhat. There are characters with well written personalities and backstories, though not much. Sydney the relic hunter comes to mind.