Another menu option under the convert menu: tag - directory name
This would open a popup with a format string field where you could then dictate the form of the directory name that contains the files you are working on. As this is only one line (the directory name) a preview is shown in the same popup and is updated everytime the format string field is updated. Once OK is clicked the directory is renamed and Mp3tag reopens that directory.
Usage: Say for example I had a folder with an Album X by Artist Y in a folder called Z. Clicking tag - directory name would let me form a directory name, say '%Artist% - %Album%' i.e. 'X - Y' and update my directory name 'Z = X - Y'. I find myself doing this often when I have downloaded a 'backup' of some music and unarchived to a folder named after the archive. Often the directory name has, say the url of the site or other junk in the name and this would fix that nicely instead of having to go to explorer afterwards and then doing manual cut and past.
Oh and great great app by the way.