One big thing google+ did that infuriated a lot of users was forced integration of all of google's properties. I think they thought "this will be great, you can just go from property to property and not have to worry about logout/login, different accounts different passwords...", but as it was unrolled it had a lot of people saying "well, i like my separation of church and state (as it were)" or "well i dislike the lack of anonymity that this creates". Instead of dealing with these concerns, and implementing a way where you could mix and match what you connected together or not (which would have been AMAZING, and i think would have been super useful), google blundered on with their plan and said to themselves "People will learn to love it. Or at least learn to deal with it." Indeed, people had none of it, and the rollout was a shitshow. So much of a shitshow that they have gone back on it and discontinued the connectivity of google plus and youtube (generally. it's kind of complicated).
I think the forced sign up and stuff really hindered it a lot. To have people use your product, they have to decide to use your product. That's why so many of us who joined early, before it was open to the public, loved and used it. My feed was a pretty massive stream of new information - friends, people I had met online, bloggers, etc. It was really nice actually. When they opened it up (quite quickly actually), and started forcing real names, forcing users to create accounts when they signed into gmail, starting fucking with youtube accounts, started having people in my contacts show up as potentials to add to my circles EVEN THOUGH THEY HAD NEVER USED G+, that's when it officially died.