Author Tract: Like many of Heinlein's novels, promotes the author's ideals of polyamory and nudism.
Badass Pacifist: Mike doesn't actually use physical force at any point in the novel; he either resists peacefully or uses his Psychic Powers to "remove" potential threats. And also lets the mob of Fosterites kill him at the end.
Bizarre Alien Psychology: The Martians are reported to have a very different way of thinking than us, including the concept of "grokking" something: to understand so thoroughly that the observer becomes a part of the observed, to merge, blend, intermarry, lose identity in group experience.
Blue and Orange Morality: The Martians are collectivist, practice eugenics (and cannibalism), and contemplate destroying other planets because it would be "a goodness". Michael comes to a sort of middle ground between the Martian and human viewpoints and morality.
But Not Too Gay: Gays are "confused" and "poor in-betweeners" who will "never be offered water" in this story. Heterosexuality is the standard. Jubal points out that men do share the "kiss of brotherhood" in the Church of All Worlds, but the man he's speaking to hastens to explain that it's "not a pansy gesture".
Cannot Tell a Lie: Anne is a Fair Witness, a legal consultant who has undergone intensive training to observe and recall events completely impartially. This skill comes in handy a few times throughout the story.
While it obviously has a televangelist aspect, some think that Smith's religion also has aspects of Scientology. Heinlein and Hubbard were friends and reputedly entered into a competition to see whose fake religion would catch on best.
Within the story, at least, Smith deliberately borrows from a televangelist religion as it was the ideal way to get his message across, but he's only using religion as a mask to deliver the true message and make it more palatable. What's even more interesting is that there really is a higher order of existence, but none of the characters are aware of it.
Other inspirations for the Fosterites are very likely Robert H. Schuller (with his lavish megachurch, syndicated TV program and "happy" message) and the Mormons (for their often-violent origin story and elaborate organizational structure).
Condescending Compassion: At first, Duke thinks of Mike as being an "innocent savage" who needs to be civilized. Jubal, however, manages to cure him of such judgmental notions.
Corrupt Politician: Jubal's glad to learn that Secretary Douglas is the sort of politician who, once bought, stays bought.
Cuckoo Nest: There's some debate whether Michael actually exercised power from beyond the grave to prevent Jubal from overdosing on pills, or if this was just a hallucination
Dark Messiah/‹bermensch: Michael is pretty nice compared to the examples on those pages, but he does conjure up Nietzsche's idea of a Dionysian worldview as opposed to Christianity's Apollonian one. The novel also contains the Fosterites (a direct inspiration for Michael's "church"), who are essentially a Dionysian Christian sect, as opposed to the usual Apollonian ways of such sects.
The water-filled sleeping sac that Heinlein imagined preceded the waterbed by several years.
There is a real-world religious group which is based upon the trappings of Mike's teachings. It's even rumored that Heinlein wrote the novel on a bet with L. Ron Hubbard (founder of Scientology), to see which one could start a bigger cult, but decided to call the whole thing off when it started getting a little too real. That story, however, has never been confirmed.
Even the Guys Want Him: Not only does Michael receive fan mail from potential female sexual partners, he also gets it from gay men and modifies his appearance to be more masculine to ward off these attentions.
Fantastic Anthropologist: Michael finds out he was one unknowingly when the Martian Old Ones download his memories of his experiences of Earth and cut the monitoring mindlink they had established with him.
For Happiness: Patricia Paiwonski, who "wanted to sacrifice herself on an altar of happiness for the world."
A God Am I: Once Mike groks the concept of human religion, he insists to everyone that "thou art God". Despite the concerns of his friends, he is not on a megalomaniacal power trip; it's a basic part of Martian philosophy.
How Do I Used Tense?: Mike's initial attempts to speak English are, realistically, full of innocent grammatical errors.
Humanity on Trial: Mike, a human raised by Martians, is sent back to Earth to grok humans and gather that information for the Martians. Humanity is never aware they're on trial, nor are the reasons made clear (though it's closest to the "dangerous neighbor" rationale), and Mike isn't fully aware himself, though some of his friends suspect something. It's revealed that the Martians already destroyed the original fifth planet and its inhabitants, creating the Asteroid Belt and making Jupiter the new fifth planet. Luckily Martians can take hundreds of years to make such a big decision, by which time humanity (with Mike's help) may be too powerful to destroy.
I'm a Humanitarian: Standard practice on Mars is to eat the dead, which squicks out a few characters when Michael talks about doing the same and refers to dead humans as "food". It's mostly symbolic, though—Martians don't feel any sentimental attachment to corpses because they believe that the body is just a vessel for the soul.
I Thought Everyone Could Do That: Michael doesn't realize at first that other people can't levitate objects. He can teach them to using the Martian language, though.
I Want My Jetpack: There are aircars, including self-driving ones, all the way through the book. (Ben Caxton is kidnapped in a robot cab.) Elevators have been replaced by antigravity bounce tubes. Jill reads books on "spools" that project the words onto a surface and turn themselves off when you put them down. TV is now 3D stereovision, watched via a "tank", but is otherwise as annoying as ever. By the time somebody uses a phone, you expect it to be a Video Phone, and it is. Of all this technology, the only one we've surpassed is the phones.
Language of Magic: When Mike attempts to explain his astonishing Psychic Powers to Jubal, he realizes that English lacks the necessary concepts, so later in the story he starts teaching people Martian. Sure enough, it turns out that all humans can do these things, but only by first teaching themselves the mental framework that goes along with speaking the Martian language.
Not Even Bothering with the Accent: In-universe version; Mike teaches other members of his nest Martian. At one point, he mentions that one member has a great grasp of the language but for reasons unfathomable even to Mike (who is at this point nearly omniscient), he manages to speak Martian with a Brooklyn accent.
Religion of Evil: The Fosterites, who are exploitative and at times violent. Not all of them are this way, but enough to enable to them to be roused to full-on mob hysteria against Mike's religion.
Sacrificial Planet: The book has a brief passage from the perspective of the Martians who adopted Valentine Michael Smith. From their perspective he's evaluating humanity, and if they don't measure up, well ... Jupiter used to be the sixth planet and Mars was always fourth.
Sarcastic Confession: When Jill smuggles the Man from Mars out of Ben's house in a large trunk, a man asks her what the trunk contains. She replies, truthfully, "A body"... he considers it a joke and lets her pass.
Science Marches On: Sadly, subsequent exploration of the Red Planet has shown it to be far less hospitable to life than Heinlein imagined/hoped. Also, most of today's astronomers reject the theory that the asteroid belt is the remnants of a former planet.
Sex by Proxy: Jubal finds out that having sex with one cult member is slightly awkward when his partner is mentally linked to all of them.
Society Marches On: Although far from a free love utopia, open relationships are considerably more acceptable today than they were in Heinlein's time, as well as homosexuality. Women are also generally not secretaries and "girls" who enjoy being patronized.
Spock Speak: Mike catches up on Earth history by reading the Encyclopedia Britannica, among other things, and talks like he's reading from a legal contract at some points. This is early on, when his grasp of English and Earth concepts is incomplete; when he doesn't know something, he sticks to the books he's read, and he has an eidetic memory.
Stage Magician: Michael uses his powers to this end at one point in the novel with Jill as his "lovely assistant". His act, despite being genuinely magical, falls totally flat because he does not understand humor and showmanship yet.
Starfish Aliens: The exact physical form of Martians is never described, but it's clear they are completely unlike humans in appearance, behaviour and culture.
Adults (who are all male) are likened to large ice ships at full mast, as the nearest human means comprehending them, and move slowly.
Nymphs (juveniles, who are all female and become male after fertilization) are like fat furry spheres.