Ditto on the waterproof -- the lack of it. Unless you are hauling a bag made for runnign whitewater, they will leak. You can use garbage bags for a pack liner or better yet trash compactor bags. Sea to Summit makes some silnylon pack liners. I use one silnylon bag for keeping socks and base layers dry and another for my sleeping bag. A pack cover will help, but can't be counted on for complete waterproofing for critical gear. A poncho works well to keep you AND the pack dry and can be helped with a waist belt (read piece of line) for wind.
Things to consider when buying a pack:
Volume-- how much space do you need?
Weight-- how much suspension do you require for the load?
Fit-- buy sized packs that fit you properly.
I go ultralight, with my base load (less fuel, water, food) at under 12 pounds. My all-up weekend trip load is about 21 pounds. With that load, I use a GoLite Jam pack. If you are going to be hauling 40-50 pounds (and you're NUTS to do that), then you need a heavier pack with a decent frame and suspension. It's just like buying boots, so I recommend getting to a store that knows what they are doing and has test loads available and trying some on and walking around the store. Look for simpler, sleeker packs with a minimum of pockets, straps and gee-gaws that just add weight. If you do a lot of bushwhacking then durability is an issue and you want heavier material. On regular trails, lighter materials are just fine with reasonable care. Every ounce you save on the pack is less weight on your feet, or another ounce of food, fuel, water, or other gear essential to the trip and your safety and comfort.
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Regards, ss.
