The hardest single part of building a software system is deciding precisely what to build. No other part of the conceptual work is as difficult as establishing the detailed technical requirements, including all the interfaces to people, to machines, and to other software systems. No other part of the work so cripples the resulting system if done wrong. No other part is more difficult to rectify later.
Frederick P. Brooks, Jr., No Silver Bullet: Essence and Accidents of Software Engineering (pp. 13), 1987, University of North Carolina at Chapel Hill