Browing Martin Fowler's web site today produced an article on Technical Debt. This concept so neatly summarizes my experiences in mistakes made maintaining enterprise software that all I need do is link to it for you to understand how I feel.

I found this article through another article on the time/functionality trade off of design activities. I actually disagree with Martin here with regard to productivity through design being purely subjective and unmeasurable. I've seen plenty of projects move faster due to consistent design, and plenty of other projects grind to a halt due to a lack of design (or at least a lack of consistent adhesion to the original design principles of the application). As with everything in software development, YMMV.