First. let me state for the record that my user name has nothing to do with the ST series.
Second, pretend I inserted Poly’s entire post here, which I agree with 100% and just want to add to.
I suspect it is difficult for anyone growing up post Star Wars to understand the miserable state of non-anthology sf in the early 1960s. Twilight Zone was good, but it was actually mostly fantasy. The Outer Limits had its moments, but for every Demon with a Glass Hand there were two monster episodes. There was Voyage to the Bottom of the Sea with its yawn nuclear sub and Venusian monster of the week. There were the other Irwin Allen shows, all more or less for kids, which had the simple Gilligan’s Island/Fugitive plot - characters try to get home and are thwarted. I was a kid, and my intelligence was insulted.
The we had Star Trek, which was the only series show that in any way resembled the good sf I read, and which didn’t treat its audience as children. Star Trek existed in a real galaxy with a real Navy-like command structure. There were of course interesting and adult themes. It was actually the first show I remember which understood you can’t go faster than light by pressing the accelerator harder. So Star Trek was absolutely revolutionary in getting sf right on network TV.
I’ll be a bit bold here and say the characters on TOS were at least as good as those in any other Trek series. More characters are not necessarily better - 3 is a perfect number in lots of ways, and 3 major characters work really well. They are archetypical - id, ego, and superego, they all fit on the screen together, and their interactions are consistent. There is also growth in the relationships over time.
Next, TOS had far better humor than any other series (and I admit I only saw a few episodes of Enterprise before I gave up.) I’m not talking just about Trouble with Tribbles, even the Doomsday Machine had the kind of humorous banter in the middle of a crisis that runs true. “Gentlemen, I suggest you beam me aboard.”
Also, TOS got real sf writers to write for them (TZ did also) and, even better, let them write their own kind of story. Amok Time is a perfect Sturgeon story. Wolf in the Fold is a perfect Bloch story. That added a lot of richness to the show.
I think Roddenberry’s naval experience made the environment of the Enterprise a lot more realistic. But I think there is another factor. I’ve been watching Have Gun Will Travel on Netflix, a show I watched as a pretty young kid, and I’m impressed about how adult it is. Roddenbery wrote for that show, and I suspect he learned a lot about writing for adults, not kids, even in a genre usually pitched to kids.