1. The NPCs aren't fantastic, but they have enough animation, and some genuinely funny or well-done voices and lines make up for it, I think.
2. It's varied and it's not. Please allow me to explain. The environment, well, it's post apocalyptic wasteland-esque, there's not much color. The wasteland and Washington D.C. style wasteland are about it. In the big term. But every building and area has so many unique details, and the wasteland is sprinkled with just enough places/towers etc. that it's not overcrowded, but enough to make you see several things in the distance you haven't before and fight an urge to immediately go explore and loot all three. Despite the fact you've been trying to get to the same location forever and have fallen victim to this impulse about 20 times in as many hours.
3. Well, for the Xbox, it generally wasn't too bad, I can't speak for the other versions. The experience is somewhat alleviated by the anticipation that 5 heavily armed Super Mutants await you behind each door. And the loading screens have some genuinely interesting images, at least for the first few times.
All in all, a game with so many details, it will keep you hooked for hours, looking for that extra Bobblehead or Easter Egg.