I once developed a specific transformation engine to convert Word documents to a given schema -- if they respect a set of rules that the transformation engine enforces.
Authors will always want to use Word; what would be interesting would be a generic tool to avoid the copy-and-paste step, but the "genericity" part is very hard...
Short of this, I built a small utility that converts rich text to Markdown: http://markitdown.medusis.com
It can be used to copy-paste from Word (or any rich text) and retains all formatting that core Markdown supports; it runs in the browser, locally (no roundtrip to any server) so no one should be able to see the content you use it for.
The goal would be: release just the basic functionality we have now with key improvements to deployment and obviously making it work in all modern browsers, then iterate from there according to customer demand with features (including things like editing from tablets and mobile devices) being prioritised by engaging with our user base.
Basically we've just spent too long working on this with absolutely no engagement with our target market (designers/devs) so we want to get it out in the open and see if it's actually worth finishing.
EDIT: If what you're referring to is the content being produced, then the fact that the markup and editing experience is so highly structured and the fact that the user is unable to put any presentation logic (fonts/sizes/styles etc.) arbitrarily in with the content unless you, as the designer, have explicitly allowed them to do it, the format is so reliable that repurposing the content to different devices is very safe and reliable -- ie. it doesn't really matter that the content editor can't preview on multiple devices, because you know that whatever they enter in, the format will be so predictable you can make it look good on any device.
The problem is that there is a learning curve to markdown, and while it's a small learning curve, it's still a hurdle. Toolbars and pseudo-WYSIWYG editors that output markdown are good baby steps. Fortunately, it's becoming more commonplace and picking up steam outside of developers with integration on sites like Reddit.