Here's my take on so called "portrait photographers" who photograph celebrities. A portrait should tell a story about a person, sort of define a part of what makes them tick. My problem with photographers who photograph actors is that generally all we are getting, is actors doing what they do best. Acting. Very few if any photographs of actors are the least bit revealing of anything and the only reason we even look is because we "know" them. Much like if we saw a picture of someone we knew or were acquainted with, we would be more drawn to look at it because we already have some sort of relationship with them. Some people confuse that with being a good portrait.
So to me, a real portrait photographer or at least a good one can take a picture of someone we DON"T know and make it memorable or breathtaking. Naturally, Migrant mother comes to mind as well as the Afghan lady with the incredibly haunting eyes. To me these are portraits. They move you and draw you in. A human empathy thing is stirred in us.
When was the last time you were stirred by a celebrity "portrait". It's mostly just unremarkable and usually as banal as the people in the photograph. Having your work on the cover of a magazine, or a book out at Christmas doesn't make you a portrait photographer just a photographer who shoots celebrities.
The freak show aspect of so called "portraits" also at times may be interesting, like going into a UFO and looking around, but not necessarily too revealing about the subject. Avedon's, western stuff is like that, as well as some native American stuff and some of Penn's work. Interesting, photojournalistic to be sure, but not really a portrait.
I find that fashion photographers trying portraiture often have plastic look to their work much like their fashion work. There is an unaccessibility to it, it's often technically good but very stand off-ish. Quite the opposite of what portraiture should be.
So my take on it is, it has to move you, transport you, and hit the empathy spot in you, then it has a chance to be a good portrait.
I'm sure on this forum, your mileage will vary.