I just started reading The Lost Painting by Jonathan Harr( 2006 Random House). In Part 1, an art historian remembers looking at painting he believes is an unidentified Caravaggio painting.
"He studied the face of the boy again, the eyes and mouth, areas difficult even for a great painter. This face, he concluded, was much livelier than the Doria version. Indeed, the entire work felt fresher and lighter in both color and execution. He detected the spark of invention and creativity in this painting, something a copyist could never achieve."
As a painter of portraits, the unnameable "thing" that I hope educated onlookers feel more than analyze, is this sense of liveliness and bravura. It's this "thing" that I look for in my own work to satisfy my own expectations.