Last night I set out to photograph my model railway crew. I knew I'd be indoors, needing a wide angle lens. I wasn't about to lug my 30 lb. backpack with it's long and zoom lenses, macro and tilt and a variety of backets.
I carried just the camera, but didn't have anything to put it in or protect it. I'm going to pick up a moderate sized over the shoulder bag, something that can lie on the ground open, with the equipment readily available, yet if it is picked up without closing it, I can safely transport the contents short distances without complete disaster.
While the backpack is ideal for carrying large weights long distances, it sucks at just about every other function. Unless you open it entirely, stuff at the bottom is hard to reach. The parts that get dirty when it's open are the same parts that go against your back when you carry it, it isn't safe for carrying unless entirely zipped, even for short distances. The weight of large lenses or my 1Ds2 cause the velcro dividers to rip out of position if there are any empty spots below - it really only works well when every container has it's full contents, then the top spot (usually for the camera) has enough support. You can't access your camera without taking the bag off your shoulder (unless you are looking at those fancy new rotating bags which seem too small for my whole bag's contents.
Something in the way of a shoulder bag that would hold camera and three lenses and a few accessories would be ideal.
It should be light in colour so lying in the sun it doesn't bake - not that this is as big an issue as with film, but still...
It should have a really good grip on the shoulder and be sufficiently padded that I can swing my tripod over the shoulder strap. It needs to be waterproof and I damn well don't want two clips to close it, one will be just fine. It better be useable while on my shoulder.