How does one drag'n'drop a directory from Explorer such that it adds to the file list in mp3tag (like "add directory")? Here the cursor shows [+] but always replaces rather than adds, regardless of modifier keys.
Thanks.
Hold down CTRL-key while you drop it.
Excellent - thanks Dano.
Suggestions for improvement: a) mention this in Help 
have the cursor shape show the difference between replace and Add.
Dano, is there also a trick to getting just the selected file(s) from Explorer into mp3tag? Here mp3tag opens all files in the same dir and sub dirs.
No that's not possible. At least there is no clean way.
You could create a playlist with the paths of the files - that can load single files. But that's not really comfortable.
If somebody wrote an explorer shell extension that creates a temporary list of the select files and sends it to Mp3tag it would be nice.
Thanks.
Other commands that would help mp3tag toward being a good playlist editor are Delete, Move up/down and Copy file list items.
Following on from this, is it possible to drag n drop multiple folders into mp3tag in one operation? Just say I want to open 50 out of 100 folders. Now I have to drag one at a time or use "add directory" one at a time, 50 times. Is there an easier way?
No multiple folders.
But you also can put the folders in a playlist and load it.
Drag'n'Drop of multiple directories is now possible with the latest Development Build. Just hold down the [Ctrl] to add them to the file view.
Best regards,
Florian
This is great! thank you Florian 
(I am loving mp3tag more and more each day.)