I like to maximize a pack's payload-to-weight ratio. The pack I used on the AT was an 8 ounce silnylon Gossamer Gear pack (the discontinued G5), and I comfortably carried 20 pounds in it, a 40:1 ratio. I amused myself by thinking of someone carrying that same ratio with a four- or five-pound pack.
That ratio caused me to rethink my packing strategy on a subsequent bike tour. I ditched my handlebar bag because of its lousy weight-to-payload ratio.
Of course, comfort is important. Thirty pounds in a heavy suspension system may work better for some than a lesser weight on a thin nylon strap.
I wish I'd found that perfect pack the first time, but I sold quite a few mistakes before I found it. At least finding the right pack wasn't as painful as the shoes.