Florians program is amazing and but I have a couple of suggestions which I think might make it a bit more user-friendly.
In the Extended Tags dialog window how about small checkbox next to each line of metadata found in the selected tag(s). The purpose being the user can tick the checkbox and have that metadata shown, if not already shown, as a column in the main program window. This would prevent the need to manually add fields in the Customize Columns window for fields that aren't already displayed. So if selecting a ton of mp3s you find they have 5 or 6 fields that you haven't already setup as columns, the checkbox would do this for you saving the time taken to cancel the Extended Tags box and manually add each column in the Customize Columns window. I've attached a jpg of what this might look like.
'Move to top' / 'Move to bottom' buttons for moving actions to the top or bottom of the list in the Actions window. At the moment there are buttons to move actions up/down one.