Major changes:
* Drupal 6 core compatibility
* CCK comptibility
* Any node type can be a newsletter
* Actions and triggers integration
* New mail backend
* Multilingual support
* Views integration
The configuration of flatcomments has been moved to the content type edit form under Administer » Content » Content types (admin/content/types). You can find the checkbox in the Comment settings fieldset (on admin/content/node-type/story for example).