Another idea might be to create a new tag for a special list (like the bullet list) but when this list is rendered a "reply-to" button is added to the entries of the list. When pressed, the reply-to button gives a blank edit form. After its posting the content is added to the list with a higher indentation level. This could allow threading. The list is still editable as a normal list. Though implementing this in a satisfying way might be hard (One needs to parse the list to find the right insertion point...) .
Added: 205a215,217
Current consensus seems to be that MeatballWiki doesn't need any of that.