They had a black woman in a respected, professional position with an officer's rank. The same with an Asian man.

They tackled race relations (in an admittedly very silly way) when no one else would touch it.

They had one of the first onscreen kisses between a white person and a black person.

Starfleet was created as a military force, but dedicated to peaceful exploration and science at the height of the Vietnam war.

Can you even imagine what today's Republicans would say about it back then? There would be a boycott of the network the first time Uhura was onscreen and it wouldn't have finished the season because there were only three networks.