First I have to say that this program is the best media library/player I have used, and the first I was willing to buy. It's just great. I am missing a feature from Media Center 11 though.
In Media Center 11, when I was editing the ID3-tag in multiple files, I could press page down and page up to move between the selected files without having to select the next file with the mouse. This was specially useful when editing name of the songs in an album, or track number. I could write a track number, press page down do move to the next file and write the track number of that file.
In Media Center 12 I couldn't get the page down and page up to work as before, so now I have to write a track number, use the mouse to select the next file, and write the track number of that file.
Hopefully you could implement the page up and page down to move between files in the tag-editor in Media Center 12 also...