In threaded message view, the subject column contains either the subject,
for threads with a single message, or a plus icon and the subject for a
thread with multiple messages.  I find this display visually confusing. 
When I have a single-message thread with an expanded multi-message thread
below it, the expanded tree looks like it has the single-message thread as
its root.

I think the threading would be clearer if the single-message threads
displayed their subjects at the same horizontal position as the
multi-message threads - ie leave a blank space where the plus icon would
have appeared.  This would make all the thread roots line up, instead of
making the single-message thread messages look like the parent of the next
multi-message node.