I'm going to say that fit is one of the most important elements. I highly suggest you find yourself a store and try on a lot of backpacks. Make sure you properly adjust them. Usually, someone in a store will be able to help you out.
Additionally, the various features you listed are useful for different activities. When buying a pack, consider what you'll be using it for and how long. For instance, my mountaineering pack has loops for my ice axe, a special pouch for my crampons, etc.
If I were planning on a day hike, I would bring my small, 11L pack. If I'm going for a few days, I might go for a 30L pack that has a storage space for my sleeping bag.
Also, you may wish to load the pack while it's in the store with the weight you estimate you'll be carrying. That way, you'll have an idea of how it will feel on your shoulders. Definitely walk around the store at that point.
One of the issues with most purchases is that you won't really know if the pack does exactly what you want until you've used it a good deal :) Live and learn! Identify what you do and don't like and eventually apply that to your next purchase.