- Angst? What Angst?: Averted. The Doctor's failure to save the paralell Earth (and being forced by the laws of time and space to abandon the versions of his friends he met there to die) strongly affects his personality from that point on - he becomes colder and shorter-tempered and alludes to it often. His Establishing Character Moment in the next season is him singing "I Don't Want to Set the World on Fire", and when getting a Mind Rape from the Keller Machine in "The Mind of Evil" he hallucinates fire and explains to Jo that it's trauma. He also experiences fire in the nightmare he has at the beginning of "The Time Monster".
- Harsher in Hindsight:
- Hilarious in Hindsight: Watching the Brigade Leader get his ass kicked by a scientist becomes even funnier when the Brigadier knocked the Master into a coma with only one punch in "The Five Doctors". Indeed, the Brigade Leader is nowhere near as skilled in combat as his counterpart.
- Moral Event Horizon: The Alternate Brigade Leader crosses this when he tries to kill the Doctor for refusing to bring him to the original world, even though doing so would destroy all of reality
- Special Effects Failure: The Primords. While the victims in the early stages of the transformation actually do look quite creepy, the fully-transformed ones look as though they've gotten lost on the way to a fancy dress party.
- The otherwise wonderfully effective cliffhanger of episode 6 is dashed by an incredibly phony-looking greenscreen effect.
- The Woobie: Sure, they're fascists, but all the Mirror Universe people die horribly. The only exception is Director Stahlman. Even the Brigade Leader and Benton count.
- Vindicated by History: It got the lowest ratings of the season on its first broadcast. Now, many consider it the best story of the Pertwee era.
- Visual Effects of Awesome: Mirror Benton's transformation scene.
- Although clearly just a color filter, the outside scenes in the alternate universe amazingly capture a dying, burning planet.
YMMV / Doctor Who S7 E4 "Inferno"