Understanding your camera and how it captures a scene, as well as a comprehensive understanding of light(ing) is imperative to a great photo and distinguishes the pros from the not so professional photographer. On top of that, when working with people, a good understanding of color correction, retouching, and makeup use will go a long way in distinguishing an experienced professional from a hobbyist or amateur.
However, even after you master all of that, there is still one important factor left to grasp if you are going to take great photographs of people (or even pets really). That would be, understanding posing and proper angles.
The wrong pose, or the wrong angle of the right pose can make the difference between you creating an outstanding photograph or your photograph ending up on a “most embarrassing photos” list on some comedy website as an example of what not to do.
Posing is an art form in and of itself. When working with professional models, the photographer has a much easier job because the model has practiced, looked at herself in photographs, knows how her body looks best, what angles suit her best, and how to convey emotion with body posture and her facial expression. The photographer is left to make sure he knows how to capture her correctly.
The portrait photographer (or head shot photographer) must take on the role of coaching his subject how to pose, finding the right angle to best present the subject (client), and making them feel at ease so the photo portrays them at their best.