I forgot to mention that a grest director has to inspire his collaborators (actors and technicians).  With the notable exception of Stanley Kubrick (and maybe Hitchcock),  I do not think that it is possible for any one person to make a great film.

Would Orson Welles be considered great if he hadn't inspired cinematographers like Gregg Toland (CITIZEN KANE), Stanley Cortez (THE MAGNIFICENT AMBERSONS) or Russell Metty (TOUCH OF EVIL)?

Gabriel Figueroa, one of the most dazzlingly inventive cinematographers that ever existed, trusted Buñuel so much that he was willing to NOT make a scene look pretty.  He totally suppressed HIS ego to bring Buñuel's vision to the screen.

Need I mention the "stock companies" of Altman, Renoir, Bergman, Allen, Ford, Hawks, Sturges and even Capra (who I don't like, but I still think is great).

These director inspire and nurture their relationship with their collaborators.  They treat them like equals and they are rewarded with work that makes them look like geniuses.  Perhaps that is the sign of a great director.  The ability to know when to keep your mouth shut and to accept the praise for work that others have done.