I would like to see the comment drop-down list turned into a textarea since comments can get quite large. For example, tracklists for mix sets may contain many lines of text, which isn't easy to edit in mp3tag. To get around this, I have to edit the tracklist is a text editor, copy the text, and paste it into mp3tag.
I would also like mp3tag to focus on a file when it's brought into the program. For example, when I right-click on a file and open it mp3tag, the file does not have focus, which means I have to manually click on it. It's an extra step for me but having the files which are brought into mp3tag have focus would really help out.