What considerations should I factor in when choosing a camera bag of a given style, and how important is each one?
For a question on the different bag styles, see: What different styles of camera bags exist, and how are they different?.
As an example list of bag qualities to get things started:
Size--both in terms of what it can hold and what it's like to carry.
Features--such as weather/rain protection, amount of padding, ease of access, places for accessories, etc.
Prices/brands--are some brands that much better so as to justify the sometimes large price differences?
Where to buy--online (direct or from a reseller) or in a brick-and-mortar store?