John Backus's Turing Award Lecture

John Backus's Turing Award Lecture was a watershed for the programming-language community because the inventor of FORTRAN, which was the dominant programming language of the day, stepped forward and said that the main stream of programming practice was flowing in a most unproductive direction. His lecture "Can Programming Be Liberated From the von Neumann Style?" was developed into a fairly technical paper which opened the floodgates for a torrent of intellectual activity that continues to this day.

While Backus's message remains as relevant as ever (simply substitute "object-oriented languages" for "conventional languages"), the technical work has worn less well.

Thanks in part to a lifetime of work by Richard Bird, the algebraic reasoning techniques in Section 12 have been developed to a point where they are much more readable and can be routinely applied to small problems; they are especially useful for developing efficient algorithmic cores. But like other formal techniques, they are not scaled up to write whole systems.
Functional programming has moved away from the point-free style inspired by APL and shown in Section 11 of Backus's paper. Current functional languages deliver more expressive power in a more readable way.

Backus's original proposals, while tremendously important to the development of a young field, are now taken for granted. Much of the most exciting work builds on two ideas brought to fruition after Backus's lecture: polymorphic type systems with type inference, and algebraic data types with pattern matching.