Mp3tag is a great, if not the best audio/mp4 tagger I use the latest (2.40) version and noticed two things:
Ogg: Disc cover import not supported? (In Ogg media or in Mp3tag?)
Windows Media audio (standard format created by Media Player): Occasionally imported covers don't "stick" - i.e. they are not properly imported, either by single file or when selecting multiple tracks at once. I tried renaming the files im Mp3tag to get rid of special characters, removing all tags... The only thing that helped was Media Player's extended editor.
Now only one thing is really missing - Matroska support. MKV is a video container, I know, but since Mp3tag supports MP4 it would be great. There is no fully fledged MKV tag editor around, so one is definitely needed