Over the years I have read about the series as well, often trying to identify just what made it so enduring and comparing the original series to its many offspring.
I suspect one element of difference is that TV series from that era were more plot driven than character driven. The Twilight Zone, Alfred Hitchcock Presents, and others of that time were noted for the quality of their writing and stories. I suspect the same of Star Trek. Sure, they had basically the same cast of characters but you never knew what intellectual idea or plot you might encounter from one show to the next. Some worked well, some not so much but they answered to the mind in a way that emotion/character driven stories do not.
That seems to contrast (to me) with the modern sensibility where it seems all series are essentially more or less well disguised soap operas with the whole attention focused on the development of the characters and their relationships with one another- see Star Trek: The Next Generation as an example.
Another striking thing about the original production of Star Trek is the inescapable negative contrast to the special effects of today. Today, you could relatively easily produce Star Trek episodes on the stage of any halfway decent high school theatrical stage. The original TV series was noticeably far away from the digital era.
Yet another notable element is how much Star Trek was of its social time. Captain James Tiberius Kirk has always been identifiable as a galactic Lothario, modeled, presumably, on the fighter pilots of the recent World War and on the glamorous pilots of the emerging commercial jets.
Star Trek saw itself as the expression of the best in America, taken to space. On the bridge of the Enterprise, you could find many cultures, many races, many species, and both genders. It was the hopeful fulfillment of the Age of Enlightenment ideal, marrying egalitarianism, tolerance, and bold action with the urge to discover.
But . . .
I am not sympathizer for the reformed Marxism of Gender Sociology but you can see what they were complaining about in the "patriarchy" of the sixties. A few nights ago, I watched the episode Who Mourns For Adonais?, Stardate: 3468.1, Original Airdate: 22 Sep, 1967.
The opening scene is on the bridge of the Enterprise. Scotty, the Chief Engineer, is smitten by a beautiful Lieutenant Carolyn Palamas, the officer responsible for archaeology and anthropology.
CAROLYN: Here's the report on Pollux Five, Captain. This entire system has been almost the same. A strange lack of intelligent life on the planets. It bugs the percentages.Poor old Scotty would be put on notice for workplace harassment in our modern era. Enterprise would constitute a hostile workplace. All the guys are joking with one another about Scott's interest in Palamas - with both Scott and Palamas right there. Scott is part of the conversation but Palamas is expected to behave as if none of it is happening. It just feels like people of the sixties trying to be enlightened and egalitarian but not quite getting it right. Or, less narcissistically, not getting it like we have ended up living it.
KIRK: Bugs the? Well, carry out the standard procedures on Pollux Four.
CAROLYN: Aye, sir.
MCCOY: Lieutenant, you look a bit tired this morning.
CAROLYN: Well, I was up all night working on this report, sir.
SCOTT: Well in that case, there's nothing like a wee bit of coffee to get you back in shape. Join me, Carolyn?
CAROLYN: All right, Scotty. Just let me give this to Mister Spock.
KIRK: Bones, could you get that excited over a cup of coffee?
MCCOY: Even from here I can tell his pulse rate's up.
SCOTT: Gentlemen. Come along, my dear.
MCCOY: I'm not sure I like that, Jim.
KIRK: Why, Bones? Scotty's a good man.
MCCOY: And he thinks he's the right man for her, but I'm not sure she thinks he's the right man. On the other hand, she's a woman. All woman. One day she'll find the right man and off she'll go, out of the service.
KIRK: I like to think of it not so much losing an officer as gaining . . .
SCOTT: Come along.
(He and Carolyn enter the turbolift.)
KIRK: Actually, I'm losing an officer.
The series is full of those gender interactions that don't match our norms today. It is neither good nor bad, just different from our norms today. Strikingly so.