Trek was always meant as a social commentary.. the sci-fi aspects were almost secondary.
I think a lot of the reason successive shows failed was they forgot that part of the show... the 60's morality play of it.
wa:do
Absolutely.
Star Trek was meant as pure social commentary; the science fiction was absolutely secondary, and, in the 1960s on television, that was a good thing. Also, with the later series, they were trying to bend newer realities we understood from science around the relatively rationally developed (in its time) but incorrect science of the original series.
But, with the original series, the social commentary was always first, and science second. They had truly powerful social commentaries, such as "Let That Be Your Last Battlefield" and "A Private Little War", commentaries on racism and the Cold War, respectively.
It was brilliant, in its day, but it was definitely not hard science fiction.