I think if you are looking at the suitability of a text format you might consider input (typing, pasting), editing (from single characters to document-wide changes) and syntax separately.

When creating HTML, I tend to use Dreamweaver's split screen (markup in one, WYSIWYG in other) editor, and choose one or other mode as it suits. If you use HTML properly, you will separate out style from structure, making it easy to control visual aspects with CSS style sheets.

When writing and editing XML, I find a lot of advantages in using a dedicated, schema-aware XML editor which integrates well with typing and editing (valid element/attribute lists on type-ahead, autoclosing tags, syntax checking).

Both HTML and XML editors I've used have syntax colouring and checking (for other file types like JavaScript and CSS too), and many keyboard and other productivity shortcuts.

There's an overhead in learning any syntax, and the more varieties you learn, the greater the overhead (although this may not be a problem for some people), and the more syntax mistakes you make (for example, I have to remember at least four separate comment syntaxes for XML/HTML, SQL, VB.NET and CSS/JavaScript).

Even in wikis where I have seen syntax like Txt2Tags, they still have to provide buttons and guides, so it is not that intuitive, and there is no real help if you make syntax errors (you have to view and find your mistake in the WYSIWYG preview, generally). I can understand your use cases, but I'm not convinced that it would be a widely-used format, especially if it cannot cope with change (like HTML and CSS have done quite successfully).