In terms of subverting expectation and critical orthodoxy this was Woody Allen's equivalent of Dylan goes electric. Anyone who wandered into a cinema to see this because they enjoyed Bananas, would be astonished, and probably felt let down. He wasn't even in it! But there was no going back.
This is an approximation of classic European theatre. It's more inspired by Henrik Ibsen than the Marx Brothers. Three sisters (ok, or Chekhov) are damaged by their oppressive, domineering mother. When their father remarries, each experiences the disturbance of past traumas. The pace is slowed right down to allow the actors time and space to capture their emotional frigidity.
The exhumation of the internal conflicts of upper middle class creatives or intellectuals became so woven into Allen's films that it eventually stereotyped him. That starts here. As does his reputation as a great writer of dramatic roles for women. Maureen Stapleton won a deserved Oscar as the second wife.
And it would eventually become Woody's standard to write for an ensemble of actors. It's easy to accept that some were not ready for this, having already experienced the metamorphosis that was Annie Hall. But it works. There's a sombre and suffocating approach- without a musical score- but it is compelling, and emotionally authentic.