One factor was "good timing", but there is no evidence for that - I'm not sure it's even an arguable point. One factor was "the inability to be anonymous", which is also true of mailing lists, many discussion forums, and even blogs with certain posting guidelines. One was the "share" feature which, as a feature, is pretty much intrinsic to hypertext.

Google+ trumped up the circles feature as a way to manage privacy settings. My (admittedly critical) reading of your post makes me think that's the only feature here worth discussing, and I don't think it alone is responsible for greatly improved conversations, especially when compared to the many other options for online discussion, and certainly not among computer scientists specifically.

A better explanation is Google's already pervasive presence (especially among computer scientists), and their monopolistic method of forcing Google+ on, for instance, GMail users. I'm not sure there are any technical decisions they made to support your thesis that their platform is uniquely suited for anything really.