Some bags need to be waterproof or close to it. That would be nice. One complaint about my harken roll top bag is that there is no inner pocket for a wallet/key/phone. If you put something small into it, it eventually works it's way to the bottom, requiring you to pull everything out of the bag to get your keys. Frustrating, but it is very waterproof.
For a backpack, I like the size and versitility of the military style packs. Lots of sizes to choose from, I just picked up one of these for a survival pack.
Edit: +1 on 'plastic' zippers. Nothing is worse than having a good pack with a rusted zipper jumping teeth and jamming. Open or closed.