Here are a few pointers which could’ve saved me both money and trouble had I known them before buying my first backpack.
First and foremost, think of a backpack as a protection for your back. Everything else is secondary. Same as shoes, a backpack’s fit will affect your gait and posture, so it should be chosen carefully - it should fit you well and should have the required features for the load that you intend to carry. If it doesn’t fit, don’t buy it.
Know what you want to carry, then find a backpack to fit it in. Understandably, we all want to have one do-it-all backpack that will work for any situation. Better to be smart and realistic about such things. Smart to evaluate your needs and see if there is gear that can have multiple purposes. Realistic to accept when there is no such gear and not try to rationalize. Smaller volumes to which additional storage or gear can be attached are generally better. A big, half-empty pack is impossible to organize and will allow you to bring things you don’t necessarily need.
The best suggestion is to bring your gear to the store and try the pack with it. This advice can be found in every online review I have read. Unfortunately, most people simply won’t do it. Too troublesome, I guess.
Caveat : bigger backpacks, supposed to carry larger and heavier items will feature sturdier frames, allowing the transfer of weight to your hips, keeping things in place on your back and protecting it from them. Some smaller backpacks forego the frame completely, relying only on padding. This is the general and rather sensible trend in the industry - the bigger the backpack, the beefier the carrying system. The problem here though is that there are plenty of exceptions in terms of carrying situations, yet there are not plenty of alternatives in terms of actual products. For example, camera gear is generally compact and heavy, however good luck finding a small volume backpack with a proper carrying system. There certainly are such products, but the available options are relatively few.
More padding is always better. Some companies make cutouts, allowing air to circulate, which makes for nice summer packs. Others make thinner padding, which does nothing except maybe slightly lowering weight and cost. The foam used for the padding is not breathable and its thickness is not going to make much of a difference in terms of heat retention. It will however provide a different level of comfort.
More pockets will make you more miserable. You will always be searching for the backpack that fits your things just right. Pockets, zippers, attachments and various convenience features should be your last concern. In the end, even if your backpack consists of one single compartment, there are ways to organize your things. In addition, any extra features add weight, cost and additional breaking points.
Speaking of breaking, zippers do fail. This happens very rarely, but when it does and you are in the middle of nowhere, your priorities will get realigned real quick.
There are no magic materials. A tougher backpack will be heavier. Choose the proper material for the required durability and protection of your items.
Manufacturer’s measurements cannot be trusted or relied upon. Firstly, companies tend to use different methods and measure different things. Some will include the volume of the pockets, some not; some will round the measurement up, others down. Second, the actual form of the backpack will define how you will have to organize your items. Some bigger items may never fit properly if the pack is too narrow, even if it has the required volume. All volume measurements should therefore be used only to give you a rough idea of which products to look at.