Its tagging spec is not very complete, in either documentation or number of tags, again, IMHO.

Yes. The open-ended (no fixed tags) nature of the comment structure is touted as an advantage, and I suppose it is, but it doesn't give a potential developer much practical information to go on -- say if you wanted to add Ogg comment support to your popular ID3 tag editor.

I've been looking through the Xiph forums to see if I can get any more background on "comments". It would seem like it would be good for someone to come up with a mini treatise on this migration issue to help with evangelism. Don't think I know enough to approach the subject. Perhaps I'll hint around on the Xiph advocacy forum.