You’re heading for a journey on the road discovering the world? Your backpack will be your best (non-human) companion so you need to make sure you are made for each other!
Here are some elements to take into account when it is time to choose one.
Note that depending on your destination and planned activities this could vary sensibly. If you are planning on travelling mainly in warm countries you should be able to travel lighter. In South-East Asia in particular it is easy (and tempting) to be clothes on the way because it is cheap and they have very cool designs; so you could leave with 2-3 T-shirts, 2 shorts and a swimsuit! However if you plan on hiking you want to make sure to have good shoes and maybe technical equipment, if you plan on travelling to Glaciers you will need to get warm clothes too, etc...
For a long period of time I really recommend choosing a backpack that can fully open rather than one that open by the top only. However if you do so you can chose to have some packing cubes that will allow you to keep things organized and easy to find, rather than having to get everything out of your bag every time you are looking for something (that of course will be at the bottom end of your bag!)
Also if you plan on camping or hiking make sure you have some extra-options to hold those accessories (straps for the tent or the map, hanger for a bottle, straps for hiking poles, etc.…)
Make sure the shoulder straps are large, well-padded and reinforced but also well-adjusted to your size.
Also you should have at minima a hip-strap; this can really make a difference as it will release some weight from your shoulders! In addition you will want the area in contact with your back to be breathable and if possible to follow the curve of your spine.
Finally make sure to arrange things smartly in your bag: heavier items at the bottom, lighter at the top.
Water resistance: either water-proof of coming with a built-in waterproof cover, you will be more than happy to have a good protection under the rain!
Color: not the most important aspect but you may opt for more discreet colors over bright ones. Best is if you can chose a color that you like but let’s be honest you won’t really have a choice as most models come in a very limited color choice.
Extra-bag: some backpacks offer an additional bag (on top or in the front) that you can strap/ clip on and out. This is very cool as it offers you a very convenient day-pack.