You really need to check out the Granite Gear Nimbus Access Trace 70. It is 70L, but it feels much smaller than that (you can always make a big pack smaller, but it's near impossible to make a small pack bigger if needed). It has a removable top lid, the best access of any pack I've seen (two zipper opening with internal compression straps), and weighs only 4lbs, 2oz. It is incredibly comfortable to carry. One of the coolest features is the utilization of a maple wood back panel instead of aluminum stays and a plastic frame sheet for it's load-hauling capabilities. The wood back panel is lighter than composite, is incased in a fiberglass resin (making it waterproof), and is surprisingly capable of carrying loads up to 70lbs. The entire pack is very durable and uses Cordura branded woven nylon as well as Cordura stretch-woven material on the back panel. Check it out: http://www.granitegear.com/nimbus-trace-access-70.html/

Next year (available February 2015) Granite Gear is coming out with a 60L version (and a 85L). It's at least worth checking out. It did take a Gear of the Year Award from Outside Magazine. I usually don't put much credence in these awards, but the mere fact that this pack rose to the top of the piles of backpacks Outside could have chosen from makes it worth considering.