We asked the people who look at headshots for a living i.e. casting directors, agents, resident directors etc., what made for a good shot, and they all came back with the same answers.
Above all, it should be a good representation of the artist, shot preferably in natural light, with a little bit of 'something' going on in the actor's eyes.
After that, things like framing and focussing can also make the shot stand out a little more. As you can see from the gallery, that is exactly the kind of shot we take.
Unpretentious, unfussy, clean photographs, shot in natural light with the artist completely relaxed so they can concentrate on getting that little 'something' into their eyes.