Not touching the legal bits, but the comparison between privnote and GMail's thing actually help me zero in what bugs me about GMail's.
From these screenshots, Google calls the note an email, integrates it with their email client, and removes some normal email controls for forwarding, etc. That violates people's expectations of email, in terms of recipients' control of their data and vendor neutrality. Those are things I don't want changed about email!
One approach is to decouple ephemeral messages from email or GMail like privnote, which avoids confusion or lock-in. You still should make precise promises, though; I think "disappearing message" is a bad way to say "we delete our copy." Seems 100% feasible but then it's just Google Privnote.
Another option is to keep it integrated but tweak it to try not to mess up email, by making the feature "expiration request" or something, and maybe many clients comply by default but don't force it, except possibly when account owner != user (work networks). That helps cooperating parties without being either vendor-specific or DRM-y. I can even imagine vendors working together on that, especially with handling of user data in focus right now. Senders can try to figure out if your recipient contacts/domains claim support for the feature so you don't try to use it when it's clearly meaningless.
A fear is that Google considers it a feature to mess with the email ecosystem by adding vendor-specific stuff, or that Google'll talk about security a lot and just happen to propose lots more solutions involving more Google lock-in. I'd like to see what comes out of Google I/O to get more of a sense.