You have the ability to have threaded (nested) comments up to 10 levels deep in Wordpress 2.7. While viewing comments on this – or any Wordpress 2.7-powered – site, you’ll notice a Reply button (assuming nested discussions are enabled). Clicking that will enable you to reply directly to that comment, keeping discussions in context.

This is probably not all inclusive, but here is the basic structure for ol.commentlist as it exists in the Default theme’s comments.php. To take advantage of the new discussion features, Chris Harrison copied this file over to his theme. (You’re more than welcome to roll your own.) For many of the elements, more than one class is used with it. Where applicable, he has chosen the most descriptive class (e.g. li.comment instead of li.alt) as the parent for child formatting.

Source: Stylizing Threaded/Nested Comments in Wordpress 2.7