For backpacks, photographers seem to typically investigate capacity, flexibility, zippers, flap pockets, tripod mounts, rain protection, axillary handles, security, and other features.
Unfortunately, by the end of the first day using their new backpacks, despite their research, far too many photographers, immediately relegate them to a closet, learning the hard way, the one attribute they never considered was whether the backpack would fit them well, allowing them to comfortably carry their gear hour after hour, day after day.
Regardless of its capacity and feature set, a photo equipment backpack has to fit the photographer's body well. A poorly fitted backpack, will result in sore shoulders, and/or a sore back. It can turn a great photo hike/walk into misery and exhaustion.
A photo equipment backpack, first and foremost, must be designed as a great backpack, with a well designed harness, and must fit its user properly.