You'll also notice in the Al Green & Stevie Wonder videos that there are people playing real instruments too. Not any synthesized computer generated shit!!! The groove is strong, like a drum machine, but it's the real machine and his talent for keeping time...not using a "beat bug" or metronome.
Back then, those guys were cutting edge, yet still pretty much accepted by all.
There are melodies, groove, riffs that catch your ear/hook points, great arrangements and an importance placed on setting a mood. Each one of their songs sounded different. No cookie cutter bullshit. There were artists that did that, because they were one-hit-wonders and had less talent, but today the cookie cutter stuff is applied to nearly everyone.