We've made a modest contribution to the cathedral: threaded comments,
committed in the xwiki-sandbox. It mainly consists in the addition of
the class "com.xpn.xwiki.util.TreeNode" used for storing a tree
structure composed of XWiki objects linked to one another via any given
property. In the case of XWikiComments, that property is the "replyto"
property. The display is managed by a CSSed list, see the addition to
xwiki.css and wiki.css, as well as a Groovy script available in
xwiki-sandbox/threaded-comments as well.