The main thing for me is that its virtually impossible to build a complex system (essentially complex that is) without having good feedback and then adjusting the design to fit. That feedback comes from testing, review and the customer. The shorter the feedback cycle the quicker you settle on the target. The XP book helped spur this on in an age where a lot of people had lost the run of themselves. However these ideas were around and practiced since software development started:

http://www.craiglarman.com/wiki/downloads/misc/history-of-it...

And before that for other activities: "Plans are nothing; planning is everything." - Dwight D. Eisenhower