What makes a great headshot or portrait?
How many times have you looked at a headshot or portrait and said "wow, that's a great shot"? Why do you think you had that reaction?
Great portraiture/headshots are not accidents - they happen primarily because of collaboration between the subject and the photographer - for a photo to look alive, convey a message or evoke a feeling, "it takes a village" - the photographer and subject need to be on the same page in terms of the "look". That "look" involves everything from expression and pose to lighting, camera angle, etc. The photographer is the director, cameraman, lighting specialist, and the subject is the actor.
There also needs to be a vision (or more than one) or concept that drives the shoot. For example, look at the Theatrical Portraits page on my website - you'll notice the lighting is very dark and moody. The shots were of the leads in a production of Sweeney Todd - a dark, evil play. In working with the cast, we talked about the look we were going for and collaborated on lighting, poses, expressions, etc. until we got the look we were going for. The lighting was setup to show high contrast, and to be somewhat underexposed. No backlighting to provide separation from the backdrop was used to further intensify the dark mood.
Contrast this with the headshots of executives and researchers where the expressions were more friendly, the lighting far less dramatic, etc.
The key is an integrated approach to achieving the goal - photographs that have a visual impact or leave an impression on the viewer.