There are many things to discuss as to what makes a great photographer.  I would ask to whom?  The link is referring to a seniors portrait photographer, which really speaks to commercial success rather than great photography. What that leads to is the definition.  I tend to believe a great photographer is one that follows his/her own path and affects / connects with people through their photography, it could be simply to show something new and different or generate a new idea, or to change the actions of people.

Most truly great artists were not recognized until many years later, the same may be true for great photographers. But if we are talking of a great photographer of commercial success, they create images that people buy!