I seriously think Dazed and Confused is one of the unsung influences on this. Dazed also is very plotless and has people mostly hopping into cars with whatever is on the radio at the time in the car becoming the film's soundtrack.
EDIT: actually QT has been doing this since Res Dogs and Pulp Fiction. The "ooga chaka hooked on a feeling" song plays out in the same way in Dogs + the Statler Brother song Butch sings along to in PF. But those moments are stretched out for long patches in Hollywood closer to the hangout quality of Dazed.