This is the only episode of the original series with scenes on the planet Vulcan. It is also the only time we visit any regular character's home planet in the present day (all trips to Earth were done via Time Travel).
This episode was written and rewritten by about fifteen or twenty different people in addition to Theodore Sturgeon. Much of the Worldbuilding we saw didn't come from Sturgeon but from D.C. Fontana, Stan Robertson, the Kellam-Deforest research people (one of whom invented a few additional Vulcan words, which unfortunately didn't make it into the show as aired) and Gene Coon with additions by the cast.
Due to the Dead: Me TV rescheduled this episode to play in the wake of Leonard Nimoy's death. While the previously scheduled "This Side of Paradise" did also focus on Spock, this episode explored his character just a bit more.
Prop Recycling: Romulan helmets are reused from "Balance of Terror", this time worn by Vulcans during the pon farr ritual. In both productions, the helmets were a de facto economy measure as they precluded the need for the actors to wear ear prostheses. Fridge Brilliance sets in when you remember that the Vulcans and Romulans were once a unified society.
The Vulcan salute makes its first appearance in this episode. Leonard Nimoy came up with it on set. Celia Lovsky (T'Pau) couldn't do the salute and had to arrange her fingers with her other hand off-screen. Hers — given with the left hand — is the first Vulcan salute viewers saw.
Arlene Martel (T'Pring) felt that her outfit looked like a maternity dress (it's an A-line with a high, Empire style waist). It occurred to her that T'Pring might be pregnant by Stonn, so she deliberately played it that way throughout.
According to Theodore Sturgeon himself, McCoy's famous final line, "In a pig's eye!", was an ad-lib by DeForest Kelley.
