What makes a great portrait ?

A busy week and a quiet one, photographically. I did just spend an enjoyable few minutes reading this post on Conscientious about What makes a great portrait. Particularly interesting was how many responders were quite dismissive of portraits as a style of photography. Maybe not dismissive, but perhaps disappointed in the fact that a portrait really can never capture a piece of the sitter's soul. Avedon said quite famously that Every photograph is accurate. None of them is the truth. At least I suppose, not the whole truth. Though the reality that a lot of portraits are perhaps more often lies. From the basic admonition to smile to the more posed or controlled expressions in formal portraits, it doesn't always or often reflect the real mood of the subject. For me a lot of this doesn't really matter. If the viewer doesn't learn something about the subject of the portrait, that is secondary for how I'm taking pictures just now. The big change and contribution from taking portraits, for me, is the interaction between the photographer and subject. I get to meet people I wouldn't normally meet. I approach people and talk to them in places I wouldn't normally. The experience of taking the pictures changes me as a person, expands my horizons and the end picture is just a side-effect.