For those unaware, for years GTalk supported using XMPP/Jabber to talk to other servers. A handle of whateverwhatever@gmail.com could speak through GTalk to someone using a different XMPP server with somethingelse@personaldomain.com.
With the introduction of Hangouts, this is no longer possible. In order to talk to GTalk users, you might be using a Google account.
Now before there's too much confusion: yes, Hangouts/GTalk users can still use XMPP clients, like Pidgin or Adium, to talk to other Hangouts/GTalk users. But such users can no longer talk to people using the global federated XMPP network of servers that are not Google.
For years I enjoyed being able to run my own XMPP server, do some interesting things with it, and still talk to my friends and family on GTalk. Now I'm forced to use Google to maintain this.
In other words, they took a well functioning standard that was successfully decentralized, federated, and working extremely well, and deliberately neutered the decentralized aspects. Now there is the nice decentralized XMPP, and Google's XMPP, separate, distinct, broken. It's a damn shame. Gtalk federation worked great for years, and it's only been with the G+era strangulation that it's disappeared.