It's immediately apparent that Barry Letts has put his definitive stamp on Doctor Who, beginning with Terror of the Autons. Letts ascended to the producer's chair rather late during Season Seven, only taking over from Derrick Sherwin for the studio recording of Doctor Who and the Silurians. As a result, Letts had very little to do with the stories selected to usher Jon Pertwee, nor with the overall tone in how they were presented.
By Season Eight, however, Letts was firmly in charge, and so Terror of the Autons is Letts's real debut in Doctor Who. It is immediately evident that the dark, intense, adult drama of Season Seven is a thing of the past. Instead of a Doctor being independent of the system and a true iconoclast, with few allies or friends at his reluctant home-away-from-home, UNIT Headquarters (which gets another of its seemingly endless makeovers in this story), we now have a Doctor who seems quite comfortable in his exile, surrounded by old and new friends. He tinkers with the dematerialization circuit in a vain attempt to override the Time Lords' control over him, but only with the intensity of that of a Sunday afternoon hobby, killing time in between inevitable trips with the Brigadier to plastics factories and circuses.
We also meet Jo for the first time, and she sets the tone for what her role will be for the better part of this story by asking The Doctor more questions in her opening scene than Liz Shaw ever did in her whole tenure on the show. So, instead of The Doctor and Liz engaging in scientific research together and trusting the viewer to assume what they're doing and what the story is about, we now get to see The Doctor outline the basics of the plot to Jo and the reasons behind his various (unfinished) experiments. So ends the era of the Third Doctor as the rebel, and thus begins the familiar image of the Third Doctor - the avuncular teacher, too busy educating and saving the world instead of standing up to it.
Oh, and The Master makes his debut, too, but I'll have more than enough time and space to get to his exploits over the next 2o-odd episodes.