I completely agree with the fov slider and thought about making a thread to ask if that would be considered, glad to see I'm not the only one.
About spectating, that will be possible and is specified in the FAQ here.
Right now it seems as though the system requirements are quite low and that most people with decent PCs will be able to run the game with very high fps. I think messing with crazy weather might change that drastically and I'm not sure in what way it could add to the experience. The idea of heavy rain is kinda cool, it being louder than footsteps so you have to look all around you since you won't hear anyone sneaking up to you. But other than that it's hard to imagine anything, a forest fire sounds lame in my opinion. The only point of that would be to push players further into the centre, which is what the damage inducing plasma field does anyway.
After everything the developers said in the trailers, I'm pretty sure headshots will do more damage than bodyshots.