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.
I'm not sure if TOS ever touted their world as a utopian society (at least not during the TV run). They said they had overcame a lot of social ills that plagued the Earth in the past, but they never said they had no problems "period".
We people in the 21st century would also say we are much more civilized than, say, 300 years ago, and have overcome many social injustices over the past few centuries. However, we would never claim that our society is totally enlightened.