I don't have all that much to contribute, but: my understanding is that Nichelle Nichols, being an African-American woman, was the most daring cast choice by the standards of the day. She played Lieutenant Uhura, a permanent bridge member and ship's communication officer (along with other specialties). Further, the kiss she shared with William Shatner is often cited as TV's first interracial kiss and was arguably one of the show's most controversial moments. 1, 2.

Another quick note that may be of interest-- I'm under the impression that Gene Roddenberry, DeForrest Kelley and James Doohan all had Irish ancestry, while Shatner, Leonard Nimoy and Walter Koenig had Jewish ancestry. (Roddenberry was the show creator and the others, leads and backing actors) Perhaps someone would like to confirm the ancestry aspect or add more.