@littlek,
IMHO, it's the depth and nuances of his words. That is one writer that has his art down. He is more like a literary novelist than almost anyone. Bob Dylan does that for me, too, as does Joni Mitchell.
I was not aware but there is a movie about him called
Leonard Cohen: I'm Your Man (2005).
More recent works such as
Famous Blue Raincoat where he sings duet with Jennifer Warnes are gems.
My very first encounter with LC was the song Suzanne, circa 1968.