Half of these things (e.g. carbon copies) are spec'd out but barely supported.
Would it not have been possible to offer a XMPP MUC as a text-based interface to hangouts, to allow ‘legacy’ users to still be connected?
The X in XMPP is for extensible. I guess the point is Skype doesn't support XMPP or federation, and Microsoft is moving towards Skype and away from their SIP/SIMPLE chat services. Likewise, Apple's promise of an open standard for interoperating with Facetime turned out to be empty.
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Edit: Found this: http://www.joecieplinski.com/blog/2013/03/30/hey-apple-where... http://www.imore.com/apple-fined-368-million-facetime-patent... http://www.techradar.com/news/world-of-tech/virnetx-slaps-ap...
And as you say - there's a bunch of stuff to support if you go the traditional route and try to bolt on more crap to MUC or <message> stanzas. We're considering just building hangouts on top of "ephemeral" buddycloud channels that just disappear when the last recipient leaves them.
XMPP also has a spec for server-side history storage, that allows for syncing to clients. I don’t know how well it is supported in reality, but understand that there are at least ejabberd modules for the server-side part and a plugin for Pidgin to view this stored history. Not perfect, yet.
I guess it is a long way till we get there, though :|