Vital Backpack components

I would like to assemble a list of 'must haves' that people look for when purchasing a backpack for their camera + gear.

Obviously, there are different requirements for different situations so, I would also like to hear what your requirements are for these situations.

My aim is to design the perfect backpack. I know what I want in one (or two) but might look into selling something should I get it right and meet the requirements of others also.

There's no such think as a "perfect backpack" IMO. I bought one from a "needy friend" years ago & haven't used it since.
BUT, what I use all the time is a belt pack. See post #32 or the Tamrac belt in Post #68.
Show me your camera bag

Review: ThinkTank Modular Belt System

light unloaded weight
waterproof/resistant material
abrasion/tear resistant material
internal frame
hip belt
not boxy and not like the "typical" camera backpack
space to put non-camera gear
adequate padding on straps and back
tripod holder
removable/repositionab​le dividers
lots of pockets for things like batteries, SD cards etc...
external attachment points for accessories such as additional pouches via webbing
looks nice
not too expensive
very durable
relatively easy access to gear yet safe at the same time (no front access to camera compartment)