Rapidly becoming one of the most popular fabrics in premium packs, Dupont-made Cordura comes in 500 or 1000 denier fibers. Aluminum comes in a variety of weights and alloys for different characteristics in flex, stiffness, and load bearing ability. Polycarbonate is usually featured as an impact resistant plastic, and as a base for framesheet materials. Delrin and carbon fiber are both featured as stays, frequently in conjunction with polyethelene sheets. Delrin is a torsionally rigid but bendable composite used to stabalize large loads in big packs.
Carbon fiber is used to the same end but is lighter, stiffer, and more expensive.
To many it's simply a matter of personal preference. A variety of foam densities are used in pack harnessing depending on pack weight and application. Be sure to research factors like breathability, particularly when choosing a pack you'll use in hot or humid conditions. Packmakers frequently apply treatment finishes to their foams to prevent mildew, bacterial growth, or UV degradation. A nice extra to look for is a wicking fabric on a harness system.