Being able to customize your backpack will not only make it easier for you to use it, but it could also get you legions of adoring fans based on your pimped-out pack. Or not.
When you look at how a pack can be customized, look at fit first. Many packs have removable hip belts to allow for a better fit; some even have moldable hip belts. These belts are filled with malleable foam. Pop it into an oven to warm the foam, mold it to your hips and soon you'll have a backpack that's customized to fit you, and only you. Also make sure that each strap of the backpack can be adjusted for the exact fit for you.
Beyond fit, look for a backpack that can be customized for how you're going to use it. External frame packs are very customizable; you can add or remove packs to the frame based on what you'll be doing. Internal frame packs sometimes have smaller additional packs or hydration packs that can piggy back onto the larger pack. Removable sections of the pack are helpful because they give you more space for gear when you need it and allow you to drop the extra weight when you don't.