What this means is that you can now use D2 to simulate most of the most popular viewing modes of the original discussion system. By dragging both the abbreviate & display sliders right next to each other you effectively remove abbreviated comments which simulates nested mode. By toggling comment retrieval order to 'Oldest First' and using up down, you can effectivel read the discussion from oldest to newest. And of course the default settings gives you the best comments first, providing a nice default view of discussions for most anonymous users (who rarely participate and we want to really show only the best comments).
You can also disable D2 in the comment prefs (the word 'prefs' in the floating dialog box) if you are logged in. Right now we're testing D2 for a large percentage of anonymous readers. As soon as we finish IE7 support we'll roll out D2 for the rest of the ACs.