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).
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).
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