TOS was a victim of the sixties. It aged pretty badly.
All types of subtle sexism. (Remember folks, women were not allowed to become Starships captain back then)
It had short term memory syndrome- hardly any reoccurring characters, or no memory of previous adventures on the ship.
The characters seemed to had have no relatives or lives outside the ship.
It was basically Kirk and his past loves and that was it.
But still, you can't beat that science fiction on that show.
as to the sexism, you're right it was a product of its era.
As for the "short term memory syndrome," almost every TV show of that era had it, it basically just means that the shows weren't doing serialized storytelling. Until very recently, like the last decade or decade and a half, that was the norm.
And if you look at TNG, it was only a little better in that regard, but the vast majority of it was stand-alone stuff.