First and foremost I'd like to thank the development team for such an amazing program, which I've been using for several years and have found it to be second to no other.
What I'd really like to see though is the ability to browse multiple folders in the same application instance, preferably through a tabbed interface. I often have to edit tags across several directories, and having multiple Mp3Tag windows in the task bar gets confusing some times.
By having the program display each directory in a tab, it would also be able to automatically update the file list according to the changes made on another tab that might also reflect on the current tab, such as when editing a file within a parent folder in one tab that is also visible in another.
Anyway, that's my suggestion, and I'd be very glad to see it implemented in a future version.
Keep up the good work!