The midpoint seems like a perfect opportunity to recommend reading an article on Joerg Colberg's Conscientious from 2008 entitled "What makes a great portrait?" Colberg's article is both an expansion of and complement to the discussion generated by the photographers who have responded to the question here on fototazo. The combination of readings will hopefully further the collaborative effort of getting to an understanding of the topic, as Colberg writes as the goal of his article in its introduction.
Colberg sent an email to a large number of photographers, as well as curators, gallerists, bloggers and others involved in the photography world and then published their responses as a collective article. Respondents include Doug DuBois, Colin Pantall, Amy Stein, Brian Ulrich, and Dylan Vitone among many others.