Simple is Better
Admittedly, I’m biased towards simplicity as this site’s name would suggest. The advantage to a more simply designed pack, though, is that you have fewer things that can go wrong. I am a fan of a simple top-loading design rather than those with zippered compartments. My main pack does have a zipper that runs across the base of the pack, supposedly to allow access to one’s sleeping bag, but I absolutely never use it (it might be the only change I would make to an otherwise perfect pack for my needs.) There are many pack designs out there with multiple compartments, zippered pockets, and supposed organization systems. In my opinion, such designs only add weight, bulk, and more opportunities for mishaps. I like a pack with one big main compartment, a couple of stretch side pockets, and a stretch pouch on the front. I also like that my pack’s lid functions as another large pocket. For some, even these features might feel “overbuilt” and unnecessary. The main point is that I’d rather have control over organization within my pack using stuff sacks and/or smart packing rather than having a pack divided into arbitrary compartments by a manufacturer. I find the former method much more versatile and useful.
Size & Volume
A number of factors will determine the size and volume of your pack. Of course, your body is the first consideration. Most quality pack designs come in differing sizes according to your torso length. Many designs also allow for slight variations with a size to account for fine adjustment in torso length. (Here’s a guide to help you determine your torso length.) You want your pack to be supported by your hips, and pelvis. Your “pelvic crest” provides a natural shelf for a pack to rest upon so that your lower body and its larger, stronger muscles, can support the bulk of the load rather than your shoulders. Figure out your torso length and look at a pack's specs, especially if buying online. Sizes such as Small, Medium, Large and even gender are less important than the actual torso length. This will usually be between 15 and 22 inches for most adults.
The volume and weight of your gear is the next factor. If possible, bring all of your gear to a store or outfitter and see which packs hold your gear better than others. Don’t forget to account for food and water with respect to weight and volume. Unless you know you are planning to get into some serious ultralight, minimalist backpacking, you’re probably going to want to look at pack volumes starting around 40 liters at the least. I suspect for most backpackers, the range of 40 – 65 liters is a general sweet spot. Of course, many variables can change that. If you’re planning to backpack in sub zero temperatures for ten days, you’re going to need a much larger pack than those backpacking in mild weather for a weekend.
Frame vs, Frameless
Water Bladder Pocket
As promised, this post is nothing more than a simple overview to get you headed in the right direction for pack choices. The best thing you can do is find a reputable outdoor gear dealer and go try on some packs. If possible, bring your gear with you, too, and load up the packs you’re interested in. Don’t be afraid to ask lots of questions and solicit advice from employees. When I purchased my first “serious” pack from REI, the employee showed me some tips on packing more efficiently that I use to this day and might never have discovered on my own.