Alternative Character Interpretation: It's deliberately left ambiguous if the Overmind is just in its manipulation and eventual assimilation of sentient races. The Overlords believe that it's a good thing, but they're basically the Overmind's slaves.
Esoteric Happy Ending: The Overlords think it's a happy ending, the idea that the children of the final generation of humans Ascend to a Higher Plane of Existence, but it still involves the extinction of humanity, and the "ascended" children are a bunch of feral psychic monsters that get assimilated into a Hive Mind. Not to mention the Overmind has done this to possibly hundreds of sentient races, and only the Overlords say this is a good thing. If a different science fiction work kept the same basic plot and cast the aliens as villains, most people wouldn't question their action as villainous.
Never Live It Down: In-verse. The leader of the island community admits that as a Jew, he never really got over his faith being discredited or his old homeland (Israel) giving up its independence so soon after it attained it thanks to the Overlords. Still, he doesn't have any particular grudge against them.
Uncanny Valley: The faces of the new generation, which resembles the type of 'average face' created by compositing many photos together and is even more expressionless than a corpse.
Values Dissonance: The book was written when feminism was in its early stages, so there are a few jarringly sexist things with the men running the show and Jean only acclimating to New Athens when she learns to appreciate the kitchen.
It also attempts to challenge racism through making "nigger" a polite description no more taboo than "republican".
The offhanded way that the extinction of everything else on planet Earth is treated as a necessary sacrifice (despite there being plenty of lifeless planets in the solar system that could've provided the necessary energy) seems a lot harsher now than it probably did in the Fifties.
Tropes in the miniseries
Alternative Character Interpretation: Did the Overlords drive Peretta to suicide with images of her dead mother, or was she simply deluded? Both possibilities seem valid, since the Overlords showed themselves willing to eliminate individuals who actively harm those they've deemed important.
Anvilicious: Karellen literally looking like the Devil, as in the archetypical Big Red Devil, though that was probably the point the filmmakers were trying to make. The Overlords of the novel did resemble classic Devils, but not blatantly so.
Narm: When Karellen finally makes his public appearance. While it's supposed to be a shocking revelation, it's hard not to laugh at something that resembles Tim Curry's Lord of Darkness.
Nightmare Fuel: When the Overlords announce there will be no more children born on Earth, there is a shot of a pregnant woman - whose stomach deflates as she screams in horror.
Romantic Plot Tumor: Ricky's love problem and Milo's girlfriend. They don't advance the plot of the Overlords in a way that couldn't be done with non-romance threads, and are mostly just there to appeal to the Lowest Common Denominator in what is a very hard science fiction story.