Our design process is actually quite rigorous. Many developers contribute to our carding sessions, and then a team of developers realizes the design in classes. It just happens that we do not produce ancillary documentation as a by-product of our design process. It's a win-win approach. The system's quality is ensured by over 1300 unit tests.
Every class has extensive unit testing that ensures it is doing what it is supposed to do.
The system's quality is ensured by hundreds of functional tests. Each test exercises the whole system, end to end, and checks that it computes correct outputs from its inputs. Our focus is on the quality and maintainability of the actual software. Smalltalk's development environment lets the software stand for itself. With proper naming conventions, well-designed classes, and the powerful Smalltalk environment, it is easy to find out how anything works.