It's standard practice in threads, when commenting on what someone else said, to quote the other person in one's reply. This creates a "back-reference" to the other post, which is useful. Yet there is something that's even more useful than that: a forward-reference, which is supported by QDL. When quoting someone else's post, you get the option of automatically inserting a footnote marker in the source post at the end of the text that was quoted. After submitting, if you hover your mouse over the footnote marker, you'll see your response in a pop-up, and if you click the footnote marker, the response will open in a pop-up window, where you can navigate within that branch of the QDL tree.
The utility of this might seem trivial at first, but it's actually worth a lot more than back-references. When other people are reading the thread, they'll see the original statement, and then they'll see your footnote marker. If the statement interests them, they can go straight from there to your post. And if somebody commented on your post and left a footnote marker, they can follow that lead as well. In a typical thread, there are dozens of different logical branches, and following one particular line of reasoning through the whole thing requires reading the entire thread (and having an incredibly good memory). With forward references, following a line of reasoning is just a matter of clicking the footnote markers to go to the next comment in the series, skipping over the off-topic posts in-between.
Allow users to insert footnotes into other people's posts.