Even just a beginning of line comment would do, at a minimum.
Essentially, the blockquote syntax, but with a different symbol, would be enough for most use cases.
Fenced would probably be the next most useful, as it would enable easy commenting out of larger sections, such as the whole of a passage between two subheadings in an essay, or a whole book chapter.
Applications that might introduce non-technical users to Markdown, such as iA Writer, Byword, and Ulysses (which has its own extensions to standard Markdown) tend to have some form of syntax highlighting for most Markdown elements, if not all.