Hello. I just started playing last week, and this is a fantastic game.
Two things I've noticed so far as a beta tester:
You cannot switch to pistol or grenade while ADS. I believe this would be easy to implement and cannot see any reasoning against it.
Also, when you are ADS you cannot change the over-the-shoulder camera. I very much like the body-stance positioning (southpaw/goofy foot, etc.) that goes along with the over-the-shoulder camera toggle, but I would like to take advantage of this when going around corners and not have to unscope to change body position/over-the-shoulder camera. The first-person view would not have to change at all; the camera would just change when you unscope.
In short: Please allow us to switch weapons and over-the-shoulder camera angle while aiming down site.