The not-so-flattering Agile analogy (not invented by me) is like trying to build a house one-room-at-a-time (where software is the "fine carpentry and dry wall" and data structures are the "foundation and roof"). "O, wow, I really like this kitchen" says the customer, "let's add a bedroom". Seems easy if you're doing fine carpentry and drywall. However, how do you do Agile foundation work and roofing? Brings to mind the parable told by the radical rabbi about the foolish man building his house on sand. The wise man built on a solid foundation.