Through walk packs have different requirements and people have various ideas on what they like in a pack. A good TW pack should be strong enough to stand up to hard treatment. It should be large enough to carry all your gear. Should be comfortable on a long walk, even when fully loaded. It should have a good harness system to distribute the weight evenly on your hips and shoulders.
Zips are an entry point for water, so fewer the better.
No pack is totally water proof despite some claims, especially nylon packs. More side pockets mean more things to snag on when in rain forests or thick bush. Pack size , a rough guide for size. For extended trips greater than 5 days 80l to 95l with additional side pockets when needed. Various companies are now making good packs for women.