What kind of backpack you decide to carry is going to be completely dependent upon what you want to carry with you. The more you decide to carry, the bigger your backpack needs to be and the more attention you need to pay to comfort. However, the bigger your backpack is, the more you will be tempted to put into it, and the heavier it will be.
One of the things I like in any kind of bag are mutiple pockets. I hate rooting around in a large bag trying to find a single item. I much prefer multiple pockets where I can easily find specific things. (It's why I adore Sherpani and Haiku purses)
Chest and hip straps are necessary if you're going to be carrying any amount of weight. My backpack weighs between 12-16 pounds, depending upon how much water I'm carrying. Most of that weight is my camera.
The other important thing is to have a bag that is just for hiking, that way all you need to do to go on a hike is fill up your water bottles, put on your boots, grab your backpack and head out the door.
Some of the things I carry in (or on) my backpack:
Camera insert for backpack
Tripod (I actually make my husband carry this)
First aid kit
Snacks (dried fruit, trail mix, dark chocolate)
WV Wildlife Pocket Guide
Lip balm (Supergoop)
Sunscreen hand lotion (Supergoop)
Empty used ziplock bags (for picking up trash)
So how do you choose? Go to a store and see what feels comfortable.