Well, over the course of the last 10 years, I have had the great pleasure to road test a number of leading manufacturers of outdoor backpacks in the harshest of harsh conditions. With a number of climbs up Annapurna in Nepal, Kilimanjaro (Kili) in Tanzania and a host of others, I am in a pretty good position to 'throw my hat into the ring' and give you a heads up.
Elephants at the base of Kilimanjaro.
Firstly, let's go over the basic features of a backpack:
Crucial to your comfort, the suspension system is the back-bone of a good backpack. Now much of my experience tells me that their is more hype than anything when it comes to suspension systems. Marketing speak does nothing for me. Hard earned brownie points in the depth of battle when you are up 5,500 meters and have spent a grueling 8 hours climbing is where I judge the product.
This is where you 'cheat' the system. A well designed hip belt will help to alleviate the weight from your shoulders and re position it to your hips and legs. I prefer those that have some basic padding, and straps that make adjustment simple and easy.
It is important that your harness is comfortable under load and fits snuggly. There should be a strap in the middle so you can keep the shoulder straps secure and not 'riding' apart.
Stabilizer straps are important as well, especially for internal frame packs. Look for packs that have upper stabilizer straps (from pack to top of shoulder harness) and hipbelt stabilizing straps. Hipbelt stabilizing straps work especially well on large fanny packs, because they of course don't have shoulder straps to help with the load.
A final component is the lumbar pad, the padding situated at the small of your back. Most of the downward force ends up at this point, so a lumbar pad with high-friction fabric is nice because it reduces belt slippage.
Most hiking backpacks have at least one main internal divider. Internal frame packs often have two compartments, one for a sleeping bag at bottom, one for other stuff. This sounds good, but in practice it tends to be a bit too restrictive and less efficient. My internal frame pack has one main compartment. This means I can arrange gear however I want, but also means I have to pack carefully -- putting oft-used gear at top or near access zippers, without unbalancing the load. Many packs have detachable dividers, which could be the best of both worlds.
The main use for outside pockets is to quickly access gear I need often. The hip belt is a great location for quick access pockets. I also like to have some rigging points for quick clips so I can clip on my camera and other items too big for pockets.
For full-size internal-frame packs, I love a a pack that allows both top and front loading. The front loading system could be a panel zipper, a "J" zipper. However, there is no set rule and manufacturers are becoming increasingly creative.