I’ve just finished watching some really old lectures of Paul A.M. Dirac speaking on quantum mechanics and quantum electrodynamics. Youtube is so neat. Somebody found an old film spool, digitized it, and uploaded it to Youtube.
I find the end of the lecture on quantum mechanics very interesting. He felt that the subject is full of problems, incomplete, and creates as many problems as it solves. He was waiting for someone to come up with something to replace it. Jump to time 49 mins to hear the discussion.