I like this idea. I've been advocating it among my more tech-saavy friends for a long time.
The terrible markup created by Word can cause a lot of problems when converting documents to different formats. Word is not a "source" editor, so you don't really have full control of the actual markup, and lots of weird things can happen to your document that you won't even notice until you try to convert it to a real format that can be used in an eReader, and you suddenly notice that half of your book is composed of a broken list or some such.
Writers who don't use the built-in dropdown menus for setting headers and whatnot in Word end up causing a lot of trouble for themselves unless they pay for someone to fix all that stuff for them later. Other writers use the "nuclear method" *shudder* which destroys all existing formatting, and then restore the lost formatting manually as a final stage. I've been to conferences where people actually recommend doing this before submitting their book to self-publishing sites like Smashwords (where I work) and the KDP program.
To me this seems like a waste of time. I understand that many writers tweak with style as a method of procrastination while writing, and that is why they can't switch software easily. I've suggested that this hypothetical "simple" editor have a copy of solitaire built-in, but it didn't go over very well.
I personally write my books in
markdown (very similar to plain text) using desktop software that looks like
dillinger.io and convert to all the formats with pandoc, the tool that the OP mentioned.
Asciidoc is also a good alternative. Markdown gives me the ability to add simple necessary markup like chapter headers, bold, italics, and quotes without dealing with the strict validation and distracting nature of (x)html. It is more than enough for people writing fiction novels. If you are writing programming textbooks you will probably want to go straight to html instead for more control over your markup, unless you have a good converter for code snippets and are happy with what it generates.
Edit:
Rather than using word tracker for keeping track of changes, I check all my books into git. You can see an example
here. If I ever work with an editor, they can fork my repo and submit pull requests with changes.
Probably out of reach for most people, but I dream of a future that a piece of software can be written to hide all the details so that your average author can use git for their book's change tracking too.