Ignore brand at first. Start with fit, and what you plan to do with it. I still hike with a GPV4 (now Gossamer Gear)...super lightweight pack that holds a huge volume but takes advantage of a lot of "double duty" feature such as no foam. Your socks and other clothing articles make your padding. Decide whether you really need ladder locks, ice ax loops, crampon patches, zippers, and all the other junk you'll find on your typical mass produced pack (even book bags at walmart!!). Hip belts! My lightest packs don't have hip belts and I don't miss them.

I want the lightest thing (that's still durable) on my back, as well as my feet, so fit and function come first. If I can find that in a branded product, great. Otherwise, it will be DIY or cottage industry gear. I've found the big brands offer nothing I could ever use.
You'll find some dandy offerings on this very website! Go here:
http://www.litebackpacker.com/ultralite-packs/index.html