Joerg and Miguel Garcia-Guzman asked a whole swath of the photographic community what makes a great portrait. All the different opinions makes for great reading.
Love this quote from Bill Hunt: “…My own take on this genre, after many years of looking at and collecting photographs, most always images of people, is that portraiture by and large fails to connect the viewer and the sitter in any sort of revelatory and meaningful way.”
Link (here).