If you like climbing or peakbagging in cooler weather, I recommend you try the Osprey Packs Mutant 38 backpack. While large enough to swallow all of your gear, it only weighs 2 pounds 15 ounces (size medium), and has a clean streamlined design optimized for rock and ice climbing that won’t slow you down over rough scrambling or more technical routes.
The Mutant 38 is an exceptionally comfortable backpack to carry with an excellent hip belt system and lumbar padding designed to transfer weight off of your shoulders and onto your hips. But it really is just intended for climbers or and not backpackers. If you want a pack that feels like the Mutant but has enough capacity for backpacking, you should try the Osprey Variant 52 which I tested last winter. That pack has the same kind of suspension as the Mutant, but has a lot more volume and external attachment points. If however you are a rock or ice climber, and you like a form fitting technical pack feels like it’s an extension of your body, this is the pack for you.