IMHO Google+'s principle problem is that it's multiple efforts all under the same name umbrella. This is confusing to users and seems to have been confusing to Google. It's a longer-than-twitter public broadcast messaging system/social network/photo sharing/single sign-on/half a dozen other things. There's some great ideas in there. Having a subscription style feed of people I want to follow, and their long-form post…
They've got some of it, but it doesn't quite work (for me at least) because of small gaps - like the fact that Chromecast works with Youtube, but not with Google+ Photos - thus in order to use Chromecast to display the wonderful video I just took, I have to essentially download it and reupload it to Youtube. Google+ Photos likely leverages the same tech as Youtube, but they're clearly being developed separately and that just seems silly.
Other people have put the fault on designers - to me this is a failure of product management and ultimately management to pull together the various forces that I'm sure exist within Google. And I don't think that was easy and that's why I say that Vic has some measure of success.
Google+ is, for me, almost really good - but it falls down in enough places that I would consider switching, for instance, if Dropbox came up with a full office suite to match Google Drive (which they seem to be working on).