Harsher in Hindsight: The episode "Eva Braun" interviews David Irving, treating him simply as a Talking Head expert on Nazi Germany and World War II. The episode was made back when Irving was still considered a reputable historian. He would later come out as a Holocaust denier. That by itself is bad enough, but what makes it worse is the fact that the show was narrated by a Jewish actor, who would later portray Holocaust survivor Mel Mermelstein in the made-for-television film Never Forget, who fought such deniers and made it an uncontestable fact in courts.
Narm Charm: Leonard Nimoy riding high on his success in Star Trek, rocking the 70's 'stache, delivering a completely serious and compelling narration of mysteries ranging from the topical, to the unheard of, to the completely ridiculous. And it is wonderful to watch.
Nightmare Fuel: The series had a music score that when combined with Nimoy's voice was enough to guarantee nightmares, especially when the episode is about nightmares.
The Amityville Horror episode includes several clips from the film. Twice we're treated to the close-up shot of a doll's eyes rolling back and glowing red.
The profile on Carlos the Jackal, to a child in the 1970s watching this show that dealt with ghosts, monsters and the like, the inclusion of this real life cold blooded killer terrorist felt terrifying as a real threat, only mitigated slightly by the footage at the end of commandos training to fight him.
The loving description of a theoretical 1906-level earthquake in modern Los Angeles in the "Earthquakes" episode.
Sequelitis: An attempt was made to revive the series in 2002 with Mitch Pileggi (who portrayed AD Skinner of the X-Files fame) as the narrator, and it lasted for 8 episodes.