When you are shopping for a rucksack, you'd better look for one with comfortable padded straps and a supportive waist strap, and a rain cover is usually necessary, which is to protect your rucksack against moisture, for example traveling in rainly days. Compare top loading bags with panel loading bags, and check out all the pockets of the rucksack also.
Besides, look for the measurements of the rucksack and check the rucksack fits the standard carry-on luggage & rucksack bag size limitation the airlines you normally fly with. Rucksack sizes are always listed in liters, which will help you estimate how much stuff you can fill inside. A good size of travel rucksack a 40L carry-on rucksack.