Re: "Google wasn't quite sure how to best coax people onto Hangouts"
Reliably syncing messages to all devices with Hangouts installed would be a great start. An Achilles heel of both Hangouts and its predecessor was the part where half the time a message would go to your phone but not your computer, or vice versa.
So one way to get people to use a messaging app would be to... have it transmit messages. (Shock. Awe.)
Regarding the "uncluttered, intuitive" UI: I for one vote for putting back text labels on buttons in Hangouts, even if that might add to the visual clutter. Google's incomprehensible little (unlabeled) pictograms add an unwelcome element of "I wonder what will happen when I press this, exactly?"