Design means more than just visual design and graphical interface. For example:

    M-x <function_name>

Executes any interactive function. Any gui-based editor will ultimately be limited by the knobs and switches that can be crammed into various graphical interface elements. Emacs doesn't have that limitation. That's where the learning curve is steeper-- you have to search for function names rather than poke around menus or toolbars, but once you've learned the function name you can map it to a key, put it in your .emacs, or call it from another function.

Much like unix, this is how emacs has managed to stay relevant despite the popularity of the IDE market. It's easy to configure and extend by defining abstractions and composing functions.