To reinforce aimless' post: Fit, fit, fit is the most important. Backpack fit is almost as individual as shoe fit! You need to get the pack that fits you and that fits your gear (which is why your gear needs to go to the store with you). Just putting weight in the pack is not a substitute for your own gear. Try not to let price be too much of a factor. Don't settle for a budget pack unless it's truly comfortable for you! Also, make sure you get an experienced clerk to help you. (S)he will help you with minor adjustments and how to pack your gear into the pack.

Check out the articles listed in the left-hand column of the home page of this site, especially this one.

Be sure to check the manufacturer's website for any packs in which you are interested. They will show the maximum recommended weight the pack can carry. If you try to carry more than the max, the pack's support mechanism won't work properly and the result will be quite uncomfortable. It's better to stay several pounds less than that.

In addition to the standard manufacturers, there are several "cottage" manufacturers who make excellent lightweight packs. These include ULA, Six Moon Designs (be sure to get the "optional" stays for more support), and Gossamer Gear, among others. Do go the store route first (so you know what a well-fitted pack feels like) before ordering over the internet, check the specifications (including maximum weight) carefully on the website and be prepared to pay return shipping if the pack doesn't fit you and your gear. Don't try for a frameless pack unless your total pack weight (pack plus everything in it, including food, fuel and water) is close to 20 lbs. or less. I personally would never buy a pack without load lifters (which connect the top of the pack frame to the top of the shoulder straps) to transfer more of the weight to the hip belt. My shoulders are quite sensitive to pressure (must be a hereditary thing as my grandkids have the same problem), so Your Mileage May Vary.
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey