The trouble with good engineering is that it relies on the sum of the parts. It's no use being a good code-monkey if you can't address all elements of the application. However, if I we're to the most important part of the Software Development Cycle, I'd say it was the wild post-deployment parties where we all get drunk and naked, and end up with Janice from Accounts behind the photo-copier. If I were to name the second most important element, I'd say it was Design.

There definitely is nothing more important in Database Application Design than the Schema.

Whoa boy! Hold your horses! Don't start that application until you are sure that the schema is correct. A simple Entity Relationship Diagram will allow the Developer and Customer tease out what is really required.