I'm an old timer TOS fan, way before "Minshara" came into the picture. Alls I ever knew was Class M, no explanation given. But way back when I was a kid...
Why M? Only thing I could think of was M being smack in the middle of the alphabet. Class M were the planets just right for human life.
Like some kind of Goldilocks thing, I thought the 26 letter alphabet was the gamut of planets. Class A planets were way too cold, Class Z planets were way too hot, but Class M planets were just right (smack in the middle).
Arbitrary? Probably. But at least I didn't invent some Minshara planets to explain the "M".
Back in the good ol' days--a.k.a. the 1970's--I assumed (and as I recall, along with other fans, too) that the "M" in "Class M" stood for "Mars", because the Star Trek Writers/Directors Guide
said, in so many words, that "Class M" stands for "Earth-Mars conditions".
From the Star Trek Writers/Directors Guide,
series created by Gene Roddenberry, third revision, April 17, 1967:
Star Trek Writers/Directors Guide wrote:
We do not have space suits available or other forms of environmental suits for hostile planet surfaces. These may be obtained for special scripts but keep in mind that we generally restrict our missions to "Class M" planets (approximating Earth conditions).
Be creative, but practical here, too. Remember, "Class M" planets will be often similar to many parts of Earth -- and with societies duplicating or intermixing almost any era in man's development. Jungle backgrounds exist on back lots, so what about primeval worlds? Or a pioneer-Indian type culture? Lovely parkland exists locally, so do unusual highly modern buildings, so do farms.
I understand the concept of most landings taking place on planets approximating Earth-Mars conditions. But will we never get to a planet where gravity or atmosphere is a problem?
Yes, assuming the right story. Also some story will undoubtedly take us outside our vessel into space for repairs or to investigate some strange object there. But generally we will avoid space helmets and weightlessness since such tales would more legitimately concern Earth's present era of space travel. The aim of our format is drama and entertainment based on character rather than on details of technology and hardware.