As you may have noticed from the release log, the Preview 2 build has a new MP3 tagger component.
The main difference from the user point of view is that most user, aka custom, tags are now saved to TXXX ID3v2 frames instead of COMM ID3v2 frames. Only the so called Musicmatch tags are still saved to COMM frames.
In addition, the Album Artist tag is now saved to the TPE2 "Band" frame to make the tag compatible with iTunes, WMP, Rockbox and some other devices & programs. The library field Band is now saved to the new TXXX(Band/Orchestra) frame.
Newcomers may now have easier time when they want to import the Album Artist and possible custom tags they have created with other programs.
The Preview 2 build supports reading legacy ID3v2 tags, but on the next tag write it will use the new tagging style.
Just remember that older versions of MC are not able to read the new TXXX tags written by the new build. If you want to be sure that the new build does not remove the old style COMM tags before you are ready to ugrade to MC13 permanently you can disable tag writing in Options > General > Importing & Tagging > Update tags when file info changes.
However, naturally it would be good if all previewers would test the new tagging system and report any problems.
P.S. I think this is a very good feature change and I decided to help the JRiver guys a bit by writing this. However, this is solely based on my personal opinions and my point of view. Jim & Matt, feel free the edit this post if you want to change something.