These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
We never see an old slavegirl. They supposedly have anti-aging treatments that prolong human life for centuries if not indefinitely. So, their looks might not fade for a long time. Of course, they're instead stuck in a life where they're used as human sex toys for multiple human lifetimes with little chance for freedom and the ever present fear of death by capricious jerks and likely conditioned to enjoy that sort of life, which is probably even more horrifying.
Curiosity and emotional control are stamped out of slaves, and they're also forcibly given birth control. So in essence, being a slavegirl is a form of physical and mental Creative Sterility.
Against all of this, there is the Gorean concept that when the right Master and slave-girl end up together, they will both be fully satisfied with the relationship - and have many centuries to enjoy it. This is the denouement of Captive of Gor, Slave Girl of Gor, and the other girl-protagonist stories, and a number of other Master/slave relationships seem to be heading that way in other books too. Essentially, true can only be found by putting the right Master with the right brainwashed sex-slave, who's clearly in a position to decide what she finds "fulfilling."
Ho Yay Shipping: Some readers find men embracing men automatically homosexual, so they get a lot out of the books.
Internet Backdraft: In 90% of the internet, a mention of Gor is equivalent to trolling, unless followed up with scathing hatred and opening the floor to burn John Norman in effigy. It is a banned topic on the StarDestroyer.net forums.
Marty Stu: Tarl is one of the most obvious ones ever.
Somewhere out there is a fan novel called Transvestites of Gor with a followup called Female Men of Gor. Fans at conventions used to joke about Podkayne of Gor, but it's uncertain whether that one actually exists.
Values Dissonance: Tarl expresses this himself throughout the first books, when he's still a Stranger in a Strange Land, less and less as he adapts. Then in the late series, he and a few other Masters clinging to their values refuse to see that the world is changing, and keep insisting that it's wrong.
Take That, Scrappy!: Even the film seems to have been aware of just how annoying Watney was, as evidenced by the ending where the ring ends up sending Watney back to Earth alone without Cabot, who finally gets to be with Talena, while Watney ends up having to explain to some cops why he's jaywalking in the middle of Los Angeles dressed in a ridiculous outfit.