My whole idea around anything like this is to generally keep it to 2 things, things that let me kill other people, and things that keep me alive. So that generally means ammo, and trauma kits. It may mean some water, especially here in AZ in the summer, but I like using a camelbak that can come off quickly if I need it to.
But if I get to the point where I can't easily go prone, and get back up with gear on, it's too much.