In a lot of ways, everything the rest of us poor fools are still trying to do on the electric came out of his playing and James Jamerson's playing on the old Motown stuff. (And a legion of others with far fewer hits)
That's a quote from longtime bassist John Willis, who I idolized (well, kinda) as a teenager, and who is a terrific player. This in response to my telling him that my band's drummer thought Beatles songs were easy to play. Not for the bass player they ain't.
Consider something as simple as Day Tripper. Starts with a moderately fast guitar riff, and then the bass comes in and doubles it. For a beginning bass player like me it's a... well, a terrific left-hand exercise. Maybe I'm just a wimp, but bass strings are heavy, and moving that fast isn't easy for a beginner.
Then you get to the chorus, and it gets worse.
Moral: Next time you're listening to The Beatles, as the song tries to slip into the "not-too-difficult listening" category, take another listen to the bass line. There's more there than you thought.