A pack has about 1-1.5 yards of material. So the weight difference in a 120 denier nylon pack with dyneema ripstop versus a pack made of 500 denier Cordora is only 0.5-1.0 of weight. The rest of the weight is in the frame, suspension, and features (pockets, zippers, etc).
IMHO, 500 denier is the minimum for abrasion resistance bottoms, climbing packs, etc) for a "no worries" pack. 250 denier works fine if you don't drag it over rocks. 120 denier with a dyneema ripstop will get holes - but only as big as the ripstop squares. Or you can just be careful not to set it down on sharp sticks :)
Heavier duty fabrics will hold up better if you carry heavier weights inside the pack. The smaller threads (denier) in light weight fabrics won't hold as much weight without the stitching ripping the fabric. If you don't carry more than 25 lbs (with occasional 30 lbs for a day or two), then 120 denier with dyneema ripstop is fine.
The frames in light weight packs (made to carry 20-25 lbs) weigh 0.5-0.75 lbs. In packs made to carry 40-50 lb loads, the frame weighs 1-2lbs.
Most of the extra weight is in the plastic lined thickly padded hipbelt, thick lumbar pad, thick back padding, and thick shoulder straps. This weight is useful for 40-50 lb loads, but only half that padding is needed for 20-25 lb loads.
The useless weight in both traditional and lightweight packs is in layers of pockets, sleeping bag compartments, side access zippers, etc. A lid pocket, shove-it pocket, and side water bottle pockets, and hip belt pockets are nice - but none are needed. You can keep your accessories in a ditty bag instead of the lid, put a water bladder (or bottles) inside the pack, and keep your rain shell on top inside the main compartment. This configuration actually works better for bushwacking and climbing.