I observed a strange behavior of the reference implementation: It seems that headings can be nested into lists, which is odd. I understand that it is technically possible to do that in html, but is that a desirable feature ?
The only possible use of that I can see is emphasizing the first word of an item similarly to the description environment in LaTeX. But in that case, we can already do it already with emphasis on the word, or we could extend the set of list types to description. That , or could be done with items like {word} blabla. or something similar.
Concerning the true sections, I would rather suggest that sub*sections always reset current list structure, and that they are always described as a level 0 blocks (i.e. up to 3 spaces followed by 1 to 6 #)