Earlier quoted context omitted.
A table of contents you don't have to manually maintain. Built-in interface to insert special characters you can't type with the keyboard. Automatic grammar checks. Auto-corrections. Macros. All of the above in one program instead of a hodge-podge of programs. Being able to send and receive documents from people with no conversion needed. No offense but you don't sound like someone who writes a lot?
* Table of contents: solved by Pandoc. * Inserting special characters: personally I don't find those "character choosing" windows to be very convenient. Hunt and peck is a slow way to type! Anyway, you can do this in Markdown with either Unicode (if you have the right keyboard) or you can write HTML escape codes (&tm; etc.) * Automatic (grammar/spelling). I don't like check as I type, but sure. Anyway, Emacs provides…
I'm a Markdown fan, but really there _are_ better solutions for book writing that maintain the ease that Markdown brings. Things like ASCIIDoc.