One thing I left out, probably because I live in the soggy Pacific NW where water sources are generally frequent: If you'll be doing desert hiking, test that pack with a gallon of water instead of a quart/liter!

I mentioned framed packs. Some folks do fine with a frameless pack (such as those made by Mountain Laurel Designs) for total pack weights up to 25 lbs. Others (like me) need a supportive frame and load lifters to transfer nearly all the weight to the hip belt for anything over 7-8 lbs. Certainly if your total pack weight (including pack) is much over 25 lbs., a frameless pack will be torture for most people. Individuals, of course, vary. Pack fit is almost as individual as shoe fit, and is just as important.