The many layers of foam. The desired effects are stability and comfort. Stability is achieved by placing most of the weight on your hips. A stable load must also ride close to your back. Comfort depends on the materials and shape of the hipbelt, shoulder straps, and back panel.
Soft foam needs a stiffener or it can create pressure points.
Finally, the straps and belt must fit your physique. Shoulder straps should curve out and away from the center of your chest, so you can breathe deeply without feeling constricted. Some packs offer interchangeable straps and belts for tailor-fitting. Consider, too, how you like to pack. If you're a stuffer, you'll prefer a top-loading bag.