The most engaging aspect of a portrait is often the subject's eyes. What they have seen and lived is beyond the realm of a photograph, but what they feel is perceptible when one looks back into them.
I find the most striking portraits to be those of intriguing people. It's not only composition and lighting, but a remarkable subject which helps to make a great portrait.
Some of the people in this portfolio I've known for years, some for mere minutes, but each one has left me with something eternal.