For my money a portrait is about recording what it is about the person that you 'pick up' on. There are occasions when you don't pick up on anything (!) in which case, make sure they're lit beautifully.
I love the idea of not knowing what to expect when I go to do a portrait. It also seems easier for me to 'get' the shot I want with a small camera.
There's an amazing variety of portraits here on APUG.