This is an episode with a number of lasting legacies. One is the debut of Sarek, Spock's father (as well as his human mother Amanda). The other is the expansion of the alien landscape with the introductions of both the Andorians and Tellarites.
|via Trek TV Podcast|
Meeting Spock's dad and actively exploring their relationship is a huge deviation in storytelling for the series, one so notable that Sarek became a character of remarkable endurance, appearing in later movies (The Search for Spock, The Voyage Home) and series (an eponymous episode in Next Generation) that only strengthened his legacy and helped shaped a new incarnation with greater depth (Star Trek, which admittedly was partially drawn from the Animated Series episode "Yesteryear").
Expanding the idea of the Federation beyond humans and Vulcans at the same time was equally remarkable. Despite the fact that both the Andorians and Tellarites had to wait until Enterprise to be revisited and expanded on, even merely their looks proved enduring (Tellarites, the "pig aliens" for obvious reasons, while Andorians kept popping up in the background for years).
four quarter analysis
franchise * series * essential * character