It seems like an enhancement of some parts of email threads rather than a reinvention: replies are identified by where they're rooted rather than a subject; a "concluded" metadata bit is present to at-a-glance delineate stale threads from completed ones; tree structure is enforced by the platform rather than the users following a plain-text convention re: top-posting and quoting; contribution, reply reading, and tree browsing all happen in a single place without involving transmission asynchrony or email clients (which mailing-list-all-the-things zealots probably do not consider an enhancement, but which history indicates most people prefer).