Look for plenty of external rings, clips, and straps for adding, clipping, and strapping things to your pack as you travel. Try the pack on in the store and don't be afraid to ask the salesperson to load it up for you. One of the TL Team even brought everything she planned to take on her trip abroad to the store, had the salesperson show her the best way to load the pack, and then walked around with it for an hour before she decided to purchase it. Make sure your backpack comes with one of these or that you buy one at the time of purchase with the correct hardware to clip it onto your pack. It doesn't hurt to also take into consideration what kind of guarantees specific manufacturers provide with their packs.
Travel is a rough game and packs take a lot of abuse.
Lifetime guarantees are becoming more and more popular and how far a manufacturer stands behind their goods should be a major consideration for you when buying. Remember, you have a choice when you buy and how and what you buy sends a big message to the marketplace. Make sure you get small combination locks for both your full size backpack and your daypack. Use them to lock your many compartments and any smaller external pockets within which you want to protect things. A good sized rain poncho that folds up into a lightweight pouch will also do the trick in keeping you and your pack relatively dry.