To put it is delicately as possible, the people who designed the user interface for the discussion list in SharePoint thought a bit differently about discussion threads than people in the Notes world.  People who use Notes Migrator for SharePoint to migrate Notes discussion databases (or applications such as team rooms that contain discussion components) to SharePoint discussion lists are sometimes pretty disappointed at how discussions look and operate in SharePoint.  You could make a case for it being better or worse, but the cogent point is that it is quite different and that can be a disruptive change.  In the worst cases, this change can really detract from an end user's first impression if their new SharePoint environment.

Quest Web Parts for SharePoint (a separate product, but highly recommended for anyone attempting to rebuild non-trivial applications in SharePoint) now contains a great solution to this problem: an enhanced Discussion View web part.  This new web part may be used in place of the standard Discussion user interface to give your users a better experience...  one that is a lot more like what they had in Notes. 

Quest Solution Architect Curtis Kelly did a nice video of this web part in action.  You can view it here [link].

You can download a beta copy of this new web part directly on SharePointForAll.com [link] and you can learn about the shipping version of Quest Web Parts for SharePoint product here [link].