Isaac Hayes was a great songwriter and singer. His song "Theme from Shaft" has phallic overtones! I own all three movies: "Shaft", "Shaft's Big Score!", and "Shaft in Africa". I'll watch one tonight in his memory.
As a songwriter myself, one thing I like about Hayes is that he sometimes spoke rather than sing. You could say he influenced rap. However, for me I often speak in my songs because singing is inappropriate. In the a cappella coda to my song "Extinct Wolf Blues" recorded last week I stop singing and speak--because the topic is death, and I will not sing about death. A few of my songs are about astro-physics and it sounds foolish to sing about that topic.
Two of Isaac Hayes' hit songs, "Hold On, I'm Coming” and "Soul Man” are enough to prove he was a genius at songwriting.