I have previously touched upon a most desired attribute of a model, and that is versatility. See "Adding A Halo Behind A Model" dated October 1, 2015. The ability to give different looks is incredibly important, and not everyone can do it. I have also touched upon portraits that can have several great images within them, depending upon the crop. See "Why I Shoot Portraits with High-Megapixel Sensors" dated July 3, 2015.
What I have not yet mentioned is another attribute that is a sign of a great model. Here is an example of a portrait of model Vivienne Michelle:
I love this image. The giveaway that we are dealing with a great model is that there is another nice image here, and it has an entirely different look:
We photographers know we are in the presence of greatness when we can get two different looks with one image.