One thing I've never understood about The Motion Picture is the use of Ilia's theme at the beginning of the movie, accompanied by a black screen for the duration of it. It's a beautiful piece of music, but the inclusion just seems to draw the film out even more.
ST:TMP was one of the last motion pictures to come with an overture attached to the front of the first reel. In the good ol' days, trailers were at the end of a movie (trailing it) and overtures were played as people took their seats, played over the curtain, which then opened to show the screen for the beginning of the film.
Wikipedia says that TMP
and The Black Hole
were the only feature films to use an overture from the end of 1979 until the year 2000 (with the movie Dancer in the Dark
It's also possible that the costumes weren't up to snuff for any lengthy screen-time duration.
Well, few of the over-the-head slip rubber masks were made to permit speech, and yes, some of the aliens were played by Trek fan, and the uniforms worn by some of the rec deck personnel had tennis shoes covered by fabric.