bbPress 2.4 introduces hierarchical replies, allowing you to pick between paginated topics or a hierarchy (similar to WordPress’s comments.)
FirmaSite is already supporting bbPress with hierarchical replies! Its free and approved by official theme reviewers in wordpress.org
Example Forum: http://demo.theme.firmasite.com/forums/
Example Threaded Topic: http://demo.theme.firmasite.com/forums/topic/example-threaded-topic/
You can read user’s reviews!