It has a pretty well defined spec. It's just not XML, and I think that's okay.
Do you have any examples of undefined or linient rules?
I'm a big fan of optional closing tags that HTML5 has. Can do stuff like:
<h2> Todo List
<ul>
<li> Do task A
<li> Do task B
<p> Dear Diary, ...
Almost as easy as markdown. Couldn't get this working with XML.