A business domain must be a meaningful chunk of business reality to its end users.
A domain model must be capable of being directly validated and explained by its stakeholders.
Users know what they do, but may not know what they know.
Too much detail can be blinding.
Users may not understand their environment or the direction they want to pursue.
Domains are complex abstractions to technologists.
A common language can help members of a group define a common vision.
A common language can help members outside a group understand the common vision.