It can be summed up just by watching his comments here in the video below in an interview with Bryan Magee.
Remember that Bryan Magee’s opening question (“But now it must have some real defects: what do you now in retrospect think that the main shortcomings of the movement were?”) refers to Ayer’s philosophy of logical positivism.
Why was Ayer a great philosopher? Because in contrast to so many other philosophers whose work has obviously been debunked and refuted, but who continue to defend their discredited ideas like the hacks they are, A. J. Ayer cheerfully admitted that the core of logical positivism was wrong (“nearly all of it was false,” he laughs), and it was badly flawed as a coherent philosophy. He moved on with his philosophy and work, without some endless, dogmatic defence of logical positivism.
That may seem like a trivial point, but it is not. How many Postmodernist or Poststructuralist charlatans would gracefully admit their theories are wrong (and they are), and move on to some other research program?