Each of the individual features of Google+ sounded great on paper. I never used it so I can't comment on the implementation. What killed it for me is that it came from Google. They'd already showed they're terrible at customer service, and terrible at service longevity/continuity, and it just seems creepy for the same company to want to do my web browser, my phone, my search engine, my social media, my web hosting, m…
What really killed it was no public API. It launched in the era of Tweetdeck and the like being major tools for consolidating social media interaction. There was no API. So to send a message to Google+, one had to specifically go across to the only application capable of talking to it, theirs. When you're already in/using Tweetdeck to post messages to Twitter, Facebook etc, why would you want to go to a whole new client to post a message?
While it seems like mostly people have moved away from using such catch-all clients for interacting (in no small part because Twitter have made it hard by their various API antics), this missed the zeitgeist at the time, and never got that critical mass going to sustain it.
If you're moving late in the market, you've really got to go where people are and bring them to you, or you've got to fundamentally solve problems to a mind blowing degree that other platforms have and can't. It did neither, even though the features like circle were really good.