Don't forget the self-documenting aspect. The manual is included, the api documentation is included, you can ask emacs which command is executed when you click somewhere or when you press a button. I recently tried to do the same thing in Intellij, pita. Not only, can you find documentation, you can jump to the source code, inspect variable values at runtime, and debug or profile everything. All of that from inside the environment.
Seems like the conceptual design helped with that too, reducing the activation energy needed for clear, interactive documentation. The code and the UI are both designed against the same set of consistent concepts, which naturally brings them together and, in turn, makes it so much easier to tie them from one to another. Simplest example: every interaction in the UI is a "command" and commands are reflected in elisp, so having a way to jump from an interaction to the corresponding code and documentation just makes sense. The documentation can be organized along the same conceptual lines as the code and the UI.
