The difference between a snapshot taker and a photographer is that a photographer knows that almost everything is under his/her control and when working with models certainly light\exposure, colour, composition, and posing. Shadows may be natural but they are also controlable through exposure, use of a reflector, fill flash, or postprocessing.
Portrait work is also attention to the smallest detail and having the eye to notice them. If you don't notice the details that make a portrait weak, then your work will never improve.
skieur