Figure out what you need to carry, for how long and get the backpack that does the job.

One issue I have had with some is that I prefer Nalgene type water bottles, and most have side pockets to hold a small plastic bottle, disposable type. I carry a water bladder as well, but I like the functionality and versatility of the larger bottles. I generally carry single wall stainless steel ones.

Assess how you need to access your gear inside to see if you are good with a top loading bag or need to open it along the front. Does it have a pocket for a sleeping bag?

Many good civilian packs are adjustable for torso length, within limits. Military ones are not.