There are also upper-echelon enhancements, like lumbar support, hydration packs, insulated liners — the sky’s the limit.
If you suffer from back pain or if you are carrying a heavy load for long distances (especially for the 30+ L packs), look for a pack with built-in lumbar support. This is often an X-shaped frame that conforms to the natural curve of your spine and works by keeping the pack close to your body and distributing the weight evenly.
Another essential feature for long-distance hikers is a hydration pack, so a backpack with a hydration sleeve is key to securely hold the hydration pack in place.
If you want to keep food cold (or hot) while you carry it, an pack with an insulated liner will be right for you.