If you are serious about it as an art form ...
take it from art people and not photo people that
This is true, but not all the truth.
To a very great extent, art is art, and what the portrait greats learned centuries ago about the visual portrayal of the human face and form still applies today. I spend as much time studying painters as I do studying photographers--the rococo artists really had it going on, and my current work is exactly what Sargent was doing.
That said, there are still elements that are unique to photography, so there are things to learn in additon to what the "Old Masters" can teach us.
To a large extent, it's because of the uniqueness of the medium: The way the camera projects three dimensional objects onto a twh-dimensional surface. Just as a painter must take into account the inherent differences of the medium when switching from oil to watercolor, the photographic artist must take into account the inherent differences of the photographic medium.
In addition, there is a difference in how the audience views the different media. I was browsing a portrait painting forum a few weeks ago where they were discussing the fact that their markets valued oil portraits much more than watercolor portraits, despite the fact that watercolor takes no less skill than oil (and in some ways, more skill).
In a similar fashion, a portrait painter gets "points" from the audience just because the likeness is good ("Hey, it looks just like me!"). and more points if the proportions are adequate.
For the photographer, however, the audience starts with an expectation of accurate likeness and proportion, so the photographer gets no points for "Hey, it looks just like me." The photographer has to pay much more attention than the painter to other aspects of graphic imaging that the painter gets, to some extent, a "pass" on.
However...the great painters get it all right.