Variable width font support.
I studied my own reading speed on a variety of documents, and found that I read variable width text about 15% faster than fixed width text. I don't mind writing code in fixed-width fonts, but if I am going to use one text editor for everything, then I very much appreciate having variable-width support.
org-mode > markdown
Org-mode allows you to write structured content within Emacs, and supports the writer with a variety of useful features and tools. Besides the rich editing, export, and extension possibilities offered by emacs org-mode itself, I find that the org format is superior to markdown for my purposes. Two primary reasons for this are that org provides syntax (such as drawers) for defining all sorts of metadata about your text, and also that org is designed in such a way that it is basically equally usable as variable-width text and fixed-width text. In particular I dislike the extent to which markdown relies on fixed-width text for its display features.
evil >= vim.
Emacs "Evil" mode pretty much provides a superset of commonly used vim functionality. Evil supports all the commonly used vim editing commands, which allows you to take advantage of vim's ergonomic design as you edit text. Evil actually improves on some vim features – for example, search and replace shows replacements being entered as you type them. Evil also provides access to the full power of Emacs just one M-x away – you get the ergonomics of vim with the power of emacs when you want it.
superior extensibility (> emacs-lisp vimscript)
Especially for a lisp fan such as myself, Emacs Lisp seems a superior language to Vimscript. Emacs Lisp is kind of like a baby version of common lisp, and supports a rich number of features on its own and with the addition of third-party libraries.
However the real advantage of Emacs in extensibility is the fact that the majority of emacs is actually written in Emacs Lisp. Emacs' Github mirror indicates that the ratio of elisp to C in the project is ~4:1. I believe most of the C stuff is quite low-level, and is related to multiplatform support, core rendering, and the like. On the other hand Vim's Github repo indicates that Vim's vimscript C ratio is ~0.8:1. Since the vast majority of emacs is written in emacs-lisp, in the emacs environment you can very easily dive in to functionality from within emacs to understand and/or modify how things work.
"self documenting" w/ C-h f, C-h k
In the Emacs Manual it is stated that Emacs is "extensible, customizable, self-documenting real-time display editor". One feature I really like about emacs is the "self-documenting" component of that description. Emacs makes it very easy to look up the docstring of a given function or command, easy to determine what a keyboard shortcut does, easy to determine what shortcuts are available, easy to determine what functionality is present from various modes, and more. In short, emacs makes it possible to spend a great deal of time within emacs without having to go online to look up how to use a given function or tool.