The Barcelona Games are unique for (at least) two reasons: it is the first edition where professional sporters were admitted and for the first time in many Olympiads there was no boycot and even South-Africa was present after abolishing the Apartheid system in 1991.
A point worth mentioning is the fact that the Olympic Montjuich stadium was build in 1929 on the Montjuich mountain in Barcelona intended to be used by the Games of 1936. Unfortunately the spanish Civil War stood in the way and the Games went to Berlin (a very peculiar decision with history in mind).