I think it would be nice to be able to have a standard for single and multiline comments that aren't rendered.

single line:

! not rendered

multiline

!!!optional-label
not rendererd
neither is this.
!!!

If the compiler returned the labelled comment blocks (as a key:string value:string collection) in addition to the compiled output that would be beneficial.

---

As an aside, this could be used as a boilerplate for generator directives... I'd considered abusing the link labels.

instead of:

[publish] # (2015-01-01)markdown
[title] # (Happy New Year)

can put it in a labelled comment block

!!!directives
{
  "publish":"2015-01-01",
  "title":"Happy New Year!"
}
!!!

You have HTML comments for this. They can do multiline just fine.

<!-- comment -->