I hope the music project can benefit from experience with the moving image genres:
1. Having the terms added to the LC thesaurus gradually made bib file maintenance complex: as terms were added, multiple edits to bibliographic records could be required if the intent is full genre representation (for example, at first there was only 'opera' from migfg, and then 'opera films' came around, and then 'filmed operas', and the across-the board use of fiction/nonfiction, while all the *subject* 'operas' is still needed to collocate operas on DVD with operas on CD and scores of operas). If all the terms were introduced at once, it would save a lot of redundant work to re-edit records.

2. Scope notes are crucial. A note similar to the genre/subject distinction found in the older music headings (concertos; for works about see concerto...) would be useful in each record. In some cases, it is necessary to look at the note for a broader term (below, 'filmed performances'); this is currently the case in subject authority records, but maybe for the genre terms, scope notes could be provided in lower-level records that address terms higher in the structure. Identical scope notes in different authority records make distinguishing among similar terms extremely difficult. In other cases, it is not clear whether terms are meant to be mutually exclusive (can one use both opera films and filmed operas? especially if you want 'opera' to be an anchor in a browse index for that item?). For example (substitute 'recorded' for 'filmed' and the relevance to music is obvious):

155 Filmed operas
555 Filmed performances ǂw g
555 Opera films ǂw g
670 Moving image genre/form guide online, Sept. 2, 2008: ǂb (opera: nonfiction or fiction work featuring opera, opera singers, or opera composers)

155 Opera films
555 Motion pictures ǂw g
670 Moving image genre/form guide online, Sept. 2, 2008: ǂb (opera: nonfiction or fiction work featuring opera, opera singers, or opera composers)

[having trouble with character limit to post these together]

For implementation (far) down the road, whatever the terms look like to how they are used, it might reduce confusion if there is abundant clear and consistent information: what the genre terms are, how to navigate authority records, how they related or coexist with subject headings, best practices for creating and editing records in Connexion, and eventually what it will mean for public services librarians who don't keep up with PSD and MARC standards who find themselves unable to find what they are used to finding through subject headings. While there is guidance on LC's site and in the subject cataloging manual, the number of questions that have come out places such as the OLAC list about genres seem to point to a benefit in additional sources information from an authoritative source. As--or before--these changes are adopted, information and explanation on genre terms needs to be publicized beyond the cataloging community--those to whom the BCC or '650'/'655' discussions are invisible or unimportant--if the system is going to be ultimately beneficial for end users.