Title aside, this song is not about the celebrated architect. Rather, "Frank Lloyd Wright" was one of Simon's nicknames for Garfunkel, who was an architecture student in college.
The "farewell" nature of the song is due to Garfunkel's leaving the duo to pursue an acting career. (More about this in the discussion of "The Only Living Boy in New York.")
The hand drums and flute are unusual, Caribbean touches. As is the fact that Grafunkel carries the vocals in a song in which Simon is saying farewell to him.
It is a pleasant-enough farewell at that, an amicable split. The "so soon" is interesting, given that the duo had known each other and worked together musically since high school. "I've never laughed so long" is also nice to hear, given the famous, or rather infamous, nature of their relationship as depicted in the general media.
The line "never change your point of view" is a nice way of saying that Simon felt he was continuously evolving, while Garfunkel seemed happily stuck in a groove. Their subsequent careers bear this out, with Simon collaborating with everyone from Brazilian drummers to avant-garde dancers....
...while Garfunkel, who had his pick of songwriters, did not choose, say, Randy Newman or Leonard Cohen to interpret, or even Cole Porter, but Jimmy Webb. Webb wrote "Witchita Lineman," "By the Time I Get to Phoenix" and "McArthur Park."
Still, Simon admits, "when I run dry, I stop awhile and think of you." Perhaps Garfunkel's reliance on classic songcraft provided Simon with some structure, when his exploratory nature could have led him to take a song almost anywhere.
Ultimately, though, it seems that their deep appreciation for each other's musical talent --and each other's sheer love of the art form-- was enough to sustain their friendship as long as it has lasted.
"All of the nights we'd harmonize 'til dawn..." even when the concert was over, or there had been no concert, the two would simply sit and play and sing for hours... and revel in the uniquely beautiful sound they made together. The laughter must have been that of pure joy.
Perhaps they only "harmony" they had was musical. Even if so, what harmony it was.
Next Song: The Boxer