Many travellers don't know how to load them properly, and you will see most of the load in a bulge, with the pack sagging down and back. Internal straps and external compression straps can remedy this, and also take the load almost entirely off the zipper. The important criteria is that it has a decent internal frame and a good waistbelt that places the load on your hips. There are times where you may have to haul the backpack around for a few kilometers looking for a place to stay. This isn't the type of equipment you should mail-order unless necessary.
Most stores have a resident expert on backpacks, who usually knows how to fit internal frames.
They should have weights in one form or another, usually small sandbags, to place in the backpack. Some stores in popular hiking areas have demo packs you can take home, or rentals that you can try out. Most backpacks are too big to carry on a three-day trek if you aren't carrying a tent and stove. If you intend to carry more than just a few kilos in your day pack, get one with a small, padded, hip belt to prevent it from bouncing into your lower back. One with an internal frame, or arched suspension system would be nice if it doesn't add to much to the weight or size.