The first of the Big 3 (Backpack, Sleeping Bag, and Tent), the mountaineering pack holds everything together and can make or break your send. Mountaineering is a very diverse sport. It can involve backpacking, climbing and even skiing all on the same rout. Trip lengths can vary from a single day to more than a month. For this you need a pack that brings together the best of everything.
A mountaineering pack has a minimal suspension system. By eliminating bulky frames the pack sits closer to your body and improves balance by lowering your profile for tricky climbing maneuvers. The hip belt and internal framesheet are removable on mountaineering packs to eliminate more weight if needed. Also, by making the hip belt removable, it helps the pack accommodate a wide range of harnesses. You can use the hip belt on the approach and remove it for the final send when you need a harness.
Mountaineering packs need to be as light as possible but also durable and waterproof to handle the demands of the rout. For squeezing between boulders on a scramble or climbing chimneys, the pack will take a lot of abrasion so a high denier Cordura is often the fabric of choice. For wet environments the pack needs a combination of waterproofing and durability where Dyneema is a better choice. Some mountaineering packs combine two or three different fabric panels over different parts of the pack. They'll have high denier panels over the areas that are likely to see the most abrasion, highly waterproof fabrics over panels with more exposure to moister and lighter weight low denier panels elsewhere to give an optimal combination of weight, durability and waterproofing.
Mountaineering often requires long approaches and lots of gear so they need to accommodate the same volume as an overnight pack or backpacking pack. Because of the large volume they're often mistaken for backpacking packs. While most mountaineering packs have no external pockets or long universal straps as a backpacking pack does, some minimalist mountaineering packs will strip out all peripherals such that all gear must go inside the pack. Although this shaves a little weight, it’s not a popular choice do to the inconvenience of getting to gear and the trouble of packing abrasive gear like ice axes and crampons. Not to mention there is no way to carry skis on this type of pack. More commonly you'll find mountaineering packs with just a few proprietary gear attachments points, meaning they won’t be too useful for anything other than the piece of gear they were designed for. Common attachment points include ice axe/Ice tool loops, crampon pouch, rope holder, and a-frame or diagonal ski/splitboard attachments.
It’s easy to assume a mountaineering pack is the best all-around backpack but it’s important to note that they are in fact highly specialized pieces of gear. It’s best to choose the type of pack that is designed for the activity you'll be participating in. If you're a dedicated backpacker, weekend day hiker or resort skier a mountaineering pack won’t work well for you. There is no particular advantage to having a mountaineering pack if you're not using it to climb mountains.