The killer feature is emacs lisp. Once you've gotten a handle on the general, day to day use of emacs and the packages in your quiver the next step is to learn the language that powers the system. This will open up an incredible amount of power and customizability. It will change what you think emacs is, fundamentally.
Writing your own functions, modes, and packages is the thing that makes emacs emacs. Right now, you configure your system. But once you learn emacs lisp you will be able to customize it. This is a huge difference.
Being able to do emacs stuff in my terminal and terminal stuff in my emacs while sharing the same kill ring and other similar kinds of state.