James Hall says it has to be:
It should have a maverick hero who is in over his head (hmm...does it have to be a HE?).
In addition, the story must revolve around a thorny issue and include an important sexual incident.
Looking back at recent mega selling books, it's hard to argue against his main points, but I'm wary of anyone who describes the plot of Jane Eyre as a rich man with issues falls for innocent young lass.
He seems to forget that the classic is not called Mr Rochester. How about: used and abused innocent young lass falls for powerful older man but flees when she discovers his dark secret. She rejects another suitor, comes into a fortune and returns when the man she loves is at his most vulnerable.
While I have doubts about Hall's literary analysis, I was impressed by Celia Brayfield's book on the same subject BESTSELLER: secrets of successful writing.
Glass Geishas, and dear reader I bought a copy.