Alright... more observations on the COMMENT tag. At this point I pretty much consider this a (rather serious) bug. Assuming you have file tagging enabled:
If you uncheck "store in file tags" for the COMMENT field, MC will wack the COMMENT field whenever you change any of the other tags. Note, this is totally unique to the COMMENT field as far as I can tell. If you uncheck "store in file tags" for ARTIST or ALBUM, MC will leave the existing tag alone when making changes to other tags. The COMMENT tag is treated differently for some reason.
This may not seem like a big deal, but other programs use the COMMENT field for their own unique tags, just like MC does. Sound Check on iTunes relies on this tag.
Now, if you have "store in file tags" checked for the COMMENT field, all is STILL not well. An existing COMMENT tag will be preserved, however any "COMMENT SOMETHING_HERE" tag will get deleted. This goes for the COMMENT ITUNNORM tag.
I hope this is not a conscious attempt to sabotage other software, or power users custom tags.
Are there any plans to do something about this, as I feel a little uneasy using MC to tag my files in the meantime.