I use Thunderbird for all my work email, but have been burned multiple times by missing unread email when using thread view because Tbird does not boldface threads with unread messages: only the individual messages when you open the thread. So I have to click on all the > to check for unread messages, and have had problems with my workflow when forgetting to do this or missing something.
Simple solution: Closed threads inherit unread status and hence boldfacing if that is used to mark unread messages. Can be generalized to other attributes of messages in the thread that require noticing.