What I found interesting when watching Mission: Impossible all the way through last year was that there wasn't a gradual shift from the 60's to the 70's style, the new decade suddenly hit the show like a brick wall with very little transition in its fourth season (or the one after Trek finished), to the point that it almost feels like a drastically different show stylistically. One that can only be called "Funky". Considering they were stablemates I'd have expected a tonal shift for Trek as well, at the very least musically.
One thing to note is that, whilst proper 70's haircuts might not have met real life military regulations it's fair to say that soldiers almost never had the right haircuts in 1970's film and TV because of actors being unwilling to trim. The New Avengers episode Dirtier by the Dozen has many fine examples, as does any Doctor Who UNIT story not directed by Douglas Camfield.