GORP is looking for insights and advice on the world's best backpacks and why we love them. Visit the Backpack Advice thread in the Gear Forums and share your insights. Of more concern is the design of the pack bag and this is largely a matter of personal preference.
Most packs designed for more than a weekend on the trail are top-loaders.
And most packs offer a bottom sleeping bag compartment, so you can access your snooze sack without hauling everything else out in the process. But if you really want convenient access to your belongings, get a pack with a zipper on the main panel either a side zipper, or for the best access, a full horseshoe. Don't be seduced into thinking a flotilla of external pockets make for a better pack than a clean, nearly pocketless design. A multitude of compartments may help you stay organized, but they add weight and complexity. They also tempt you to add weight around the perimeter of your pack, instead of inside and close to the back where it is least likely to impede movement.