Whatever you choose, do not to go cheap. During high school my book backpacks would always fail. Either a shoulder strap would rip off at the top, or the zipper would derail, or the fabric would rip or fray or thin. School backpacks are just not made for anything other than light loads.
Remember that Made in China = Cheap! Buy American. We need to support our own infrastructure AND we have quality control in this country.
Make sure the fabric is at least 1000 denier (note that denier is a weight rating not a strength rating) cordura; better yet get the American-made Allied Signal Fibers Tru-Ballistic 1050 denier fabric; it meets U.S. Military Specification #MIL-C-12369F-GL. It is a smoother fabric than cordura, which looks textured. Don't get the foreign 1680 so-called ballistic fabric. 1680 is cheaper, but it is not as rugged. You don't want your backpack to fail you anymore than you want your boots to. A BOB needs to be a quality bag.
I would PLAN to pay no less than around $100 for a quality BOB, and make sure you inspect its construction to be sure that it will NOT let you down. What I mean by this is find out what materials it is made with, and whether or not Velcro is what you want, etc. You might want those superfluous pockets, you might not. Make sure it is large enough that there is some room left in it when fully packed. This reduces strain on the stitching and fabric, and eases getting at stuff, and also allows for an extra water bottle and stuff found in transit. There should be a reason for every feature. Notice if the stitching is up to par or not. Turn the bag inside out and look at it.
It must have comfortable shoulder pads and waist pad. It should also have a sternum strap. External attachments points are great for strapping and also for pouches. You should be able to attach bulky items to the outside, like a tarp, iceaxe and snowshoes, or hatchet.
After you buy it, load it up and walk a mile or two with it on. Run with it on. Jump with it on. Give yourself time to return it to the store if you don't like how it rides.
Be sure to go with a reputable, AMERICAN brandname that offers a lifetime warranty. But even this is no guarantee that it will not fail, so know what you are buying.
I wouldn't worry about if it looks military, unless you plan to use it in an "enemy" country. It is a BOB, so it probably should be inconspicuously colored, although it doesn't need to be camouflage.
Also, heat seal all loose edges on the inside nylon (if any) with a lighter to prevent fraying. Get a large ALICE pack waterproof liner to keep everything dry, or just use a heavy duty trashbag and tie it off.
Bottomline: Your BOB should be chosen to best suit your particular bug-out plan. Don't compromise when it comes to such a vital piece of gear as a BOB. Insist on quality: functionality, versatility, and ruggedness.

Anything else?