You really need to define the specific camera and lens. My camera with standard trail lens and accessories run about 8 pounds, so I go with a heavy Lowepro top-loading camera bag worn over one shoulder.
You also want to define your typical photo targets. As an example, if you shoot landscapes, then you can unzip the top, pull out the camera, line up the shot, and click. Others of us go after wildlife, so we need a camera bag that can be flipped open instantly, the camera pulled out, and click. I bypassed the top flap zipper on my Lowepro with a Velcro strap for speed of entry.
Also, you don't want your camera bag hanging down too far if you intend to be crossing any high-water streams. Good camera bags are weatherproof, but they are not commonly rated for submersion. While crossing one deep stream last summer, I had about two drops of water get into the camera compartment, but I could deal with that quickly once I got across.