This is the big thing, the whole "join a hangout" never got me interested. I don't want to communicate with some random guys on the internet, but passively joining a Hangout with some interesting people would be great.
This Google Hangout thing is the biggest success story out of mediocre Google Plus. They appear to have found the next evolutionary entity after YouTube. It fits so nicely and it has a huge potential as a game-changer beyond TV viewership.
Add the availability of green-room technology for average computer user and one can imagine the possibilities.