For several years I've been convinced that one of the core concepts of a system that should be modeled is the business domain. Being able to tell a common story using language, diagrams, and actual code that is focused on the business problem being addressed is one of the incredibly powerful capabilities of object-oriented technology.
This approach is being increasingly known as "Domain Driven Design". The term comes from an incredibly good book by Eric Evans. While browsing the Internet I came across a pretty good video that describes the benefits.