I think a portrait must be consensual, that is, the subject knows they are being photographed and is participating in the process (even momentarily). A non-consensual photo of a person, to me, is a snapshot or a street photograph.
I'll take what's behind door two!
Hats off! Great work!
The best portraits are those where the neither the photographer. nor the sitter sublimate their ego, but find some common ground, and get to know each other. YMMV, of course, but that's my experience... I need to allow them a moment, and then they may offer me the gift of their expression, and I hope to find it precisely when it's offered...
I think a good portrait tells the story of a person...reveals something... even if they are not in the frame...
There is an annual portrait competition in Sydney called Head-On. This year one of the accepted portraits was of a stark, bare, empty room in a hospice (apart from the incongruous full length wallpaper on one wall of a chocolate box forest scene) where the photographer's grandfather had died. There was an interesting discussion about what makes a portrait during a floor talk I attended a few weeks ago. Sorry I don't have the answer! The organisers also said they had an interesting email exchange with one phtographer who insisted his insect "portraits" be considered.
I must admit as I went around the walls the first time it didn't register that there was no person in the photo. The power of caption and context.