I miss an option to make the current thread (the thread which belongs to the currently selected message) marked as read (and move to the first message of the next thread). Same for navigating without marking it read. And this in both directions. For Opera Mail the shortcut "M" marks the current thread read, "N" marks the current thread read and navigates to the next. (but I understand N is already used).
Depends on #3297 <http://www.thewildbeast.co.uk/claws-mail/bugzilla/show_bug.cgi?id=3297>