One small but (sometimes very) annoying flaw:
When for some reason the file to be tagged cannot be opened for writing (for instance if it is currently playing ...), MP3TAG displays an error message (OK) and discards all the changes (NOT OK!!!).
I would like/expect MP3TAG to leave the input area and selection untouched, so that I can close the blocking applications and then repeat my attempt.
Apart from that: nice, nice, nice ...