On the trail, I carry my camera on my chest in a ThinkTank holster - strap over shoulder and waist strap to stabilize - (20 V2.0 works for the lens of the day). I keep the camera strap out, and around my neck so that I can pull the camera out of the holster any time, short notice, as opportunities arise and yet otherwise have the camera secured if I do something clumsy. I carry a small (most days) backback as well, with hydration, other daypack needs, and the second string lens for the day, unless it is just a short stroll.
But mainly, I don't have to breakdown everything to get off a shot. I can just pull 'er out and shoot, and I take a lot more pictures that way. Knowing the camera is right there means your vision is always scanning for an image. Yet she is secure in her holster (and comfortably so, for me) for steep ups and downs. Moreover, the worst tumbles I take in rough terrain are descending wet or slippery so the heels go out forward and one lands on ones butt or back such that the camera/lens unit would be somewhat exposed to trauma if on my back. I like it up front where I can defend it with my hands and arms. The second lens du jour is still back there, but less exposed than if canted onto the camera, and alway in a semi-hard case.
The real deal for me looks like the Thinktank backpack with the connection kit to have your holster on your chest - but I haven't gotten there yet.
Thanks for all the interesting ideas.