For many years, threaded comments was a frequently requested Blogger feature.
That option was finally added in 2012. When Blogger added Google+ hosted comments in 2013, threaded Google+ commenting was later added.
Threaded comments, provided as an option in Blogger hosted comments, have significant requirements. Google+ hosted comments have additional requirements - and predictably, threaded comments in Google+ hosted comments have still more requirements.
We have explored the ownership issues of Google+ hosted comments, and similarly, visibility issues.
Google+ Comments involve issues of Google+, and of commenting in general.
In making Google+ hosted threaded comments work properly, one must consider the requirements of Google+, Google+ Comments, and Threaded Comments together. And do not overlook the need for authentication - and proper cookie filtering required for commenting, in general.
Finally, to have a commenting based conversation, you need a relationship between the two parties in the conversation. That relationship, in Google+, is provided by Following, or Circling.
Non public Google+ comments involve circle membership.
Visibility of a Google+ hosted comment, when not published Publicly, requires that the person publishing a comment to have Circled anybody who will read the comment. The ability of someone to reply, in a threaded comment, then requires both parties to have Followed, or Circled each other.
Additionally, since comments (in Google+ hosted commenting) are treated as posts (in Google+ Streams), comment replies (in Google+ hosted commenting) are treated as comments (in Google+ Streams). The publisher of a Google+ Stream post (Google+ hosted Blogger comment) owns the Stream post (Blogger comment) - and controls the settings for that post (comment).
Google+ Commenting involves membership and control issues.
Ownership of a Google+ hosted Blogger comment is the opposite of a Blogger hosted Blogger comment.
With the latter, all comments are owned by the blog owner. With the former, each comment is owned by the comment publisher - except replies are owned by the person publishing the reply (who might be the blog owner, or just as likely a third party).
This complexity is one possible reason why comment moderation is a community based process.
An Important Update
If you did not use a Blogger / Google account when you Followed this blog, years ago, you are probably not Following now . During the past...