Excellent, George.

Your deciding factors are pretty accurate. I add that it is likely a combination of two or three of your five.

It's not viral exactly, but reactions to my (almost daily) comments in the New York Times verify your list.

The subject matter is likely key (the winning film at the Oscars is almost always related to the subject).

Originality (sometimes) hits the jack-box. There has to be just enough familiarity to catch fire.

Best to you.