Equipment and Process (why it's so important)
Photos are only as good as the weakest link that created them. (Unless the photographer got lucky.) This includes the quality and consistency of the lighting in addition to the quality of the cameras and lenses. It also includes the skills of the photographer: mastery of lighting, attention to detail, expertise and artistic ability with Photoshop other post-production programs; and the people skills that are so necessary to evoke the desired expressions in headshots.
(But data capture is only the first step.)