The block based layout reminds me a lot of some of the desktop publishing software I used to use in the early-mid 90's. At the time, it served well the purpose of integrating multiple media types into a single document, but it was a bit of a clunky way to edit. That's my concern with Sir Trevor.

The nicest (for me) aspect of Markdown, is that it removes the need to mess around with anything other than text to produce a formatted output document. Sir Trevor seems to remove that benefit, for limited upside.