The Klingon language spoken in the film is actually just gobbledygook made up on the spot. Later, when Star Trek III was in production, the filmmakers decided to construct an entire language out of this handful of lines. (Source)

The famous main theme of Star Trek almost never made it into the movie. The first version was met with a negative response by producers, so composer Jerry Goldsmith went back to work and produced the piece we know and love today. (Source)

Leonard Nimoy originally refused to reprise his role as Spock, but director Robert Wise convinced him to change his mind after his children told him that Star Trek wouldn't be the same without dear old Spock. (Source)

Although the planned series that The Motion Picture's script is based on never saw the light of the day, many of its ideas would be used in Star Trek: The Next Generation, which would debut in 1987. (Source)