A lot of our users simply don't do much styling of text because they feel it is too cryptic to do so. This is particularly noticing and detrimental to documents created in Phriction as they just end up being plain text documents.
The Wiki style markup was designed back when there wasn't an easy way to do wysiwyg editing within the browser, but those days are long gone. There seems to be little need for a markup language these days as full wysiwyg is done easily on the web now. It would simplify things as there would no longer have to be a preview section.
Please consider moving away from having to do wiki style markup to style documents and just make it full wysiwyg editing within Phabricator. It would make our lives, and our text, much richer. :)