Google Talk was xmpp based and had many of the features
+Timo Reimerdes mentioned and I miss too.
I still do not understand why Google isolated their users with hangout. Just because all others do it the same way?
Image you need different different phones to call people who have contracts with different providers, or different mail programs for each mail provider.
That is the messenger world today.
Multi protocol clients are a workaround but one protocoll used by everybody would be the solution - xmpp.