Always struck me as funny that TOS was supposed to be utopian in values and show the best of humanity when McCoy is such a racist arsehole to Spock all the time.
And everybody's just cool with it
. Imagine if he walked on to the bridge and talked that way to Uhura or Sulu about their race.
I prefer TNG's approach in that humanity have progressed further in ridding themself of prejudice and conflict. It makes for stranger viewing and better escapism.