I find the 'Replace' and 'Replace with Regular Expression' actions invaluable. But it is distracting to have the dialog close after each replacement and need to be set up from scratch for the next step in the process. Office applications typically make the Replace dialog persistent, and an option to do this in MP3Tag would be useful. So far as I have been able to discover, this capability is not available at present.
I suggest that a tick box be added to the dialog (there is room) to provide an option to leave the dialog open, with all previous settings still in place, when the replacement has been performed. Ideally, in this mode the dialog would be non-modal, so that the target selection as well as the dialog settings could be changed as necessary for the next replace action. With this feature, the dialog would of course need a 'Close' button.
Another useful feature, which is available elsewhere in the program (and is common in search dialogs in office applications), would be a drop-down list of, say, the 10 most recent entries in the 'Original' and 'Replace with' text boxes. This, combined with a persistent and ideally non-modal dialog, considerably reduces the number of and simplifies the steps needed when one is, for example, replacing Roman with Arabic numerals in a particular field.
These changes would be useful in the 'Replace with Regular Expression' as well as in the 'Replace' dialog. I find that I often have to experiment with regular expressions; a persistent, non modal dialog that 'remembers' previous entries would allow a previous step to be undone and the expression to be adjusted for the next attempt.