What makes a great portrait?

The first thing I notice in a person is always their eyes. In a portrait that's the first thing I notice as well. My idea of a great portrait is when the eyes catch your attention very first. I like to be left wondering and this is caused by an interesting look in the eyes. The National Geographic photograph of the Afghan girl does the best job of leaving the person viewing it wondering what was she looking at and why does she have such a distressed look about her? I also like portraits that are obscure. From a weird angle, not head on and with abnormal lighting.