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.