vi is great for people that need an fast flexible editor with great regexp support and speed... there's not a whole lot you _can't_ do in an incredibly short amount of time once you learn the commands.The thread also introduced me to this classic: "Ed is the standard text editor." (The greatest WYGIWYG editor of all.)
emacs is great for people that want to do nothing else but live inside their editor.. eat inside their editor.. have kids inside their editor.
extensibility is overrated.
For more information, see the relevant Wikipedia page.