Inacessibility leads to dehydration. Solid construction using standard components . Double cloth or other reinforcement where straps attach or where lashing points are provided. Compression straps across front-loading zippered major compartments. This cuts down on the possibility that the edges of the panel will stretch and pull apart the cloth weave.
Some expensive packs use proprietary hardware.
Generally, the heavier your load the more substantial your pack will need to be and along with that usually comes more weight. One advantage of internal frames is generally lower weight for the same space. However, few or none are totally waterproof, at least not at the seams and comparment openings, so a rain cover is a necessity. The combination of properly fitted rain cover, water resistant fabric, and waterproof packaging provides good assurance agains moisture. You can pay a lot more but it isn't necessary for normal backpacking activity.