The article says it's a mystery but then nails it:
"In line with Google’s vision to organize the world’s information, the focus on Google+ seems to be shifting to content more than relationships. ... I wonder if Google is more interested in being Reddit (the front page of the Internet), than it is in being a Facebook/Twitter clone (what your friends are up to)."
Problem with that is that G+ is an even worse version of Reddit than it is a Facebook clone.
Reddit already has (a) anonymity; (b) a working karma system; (c) proper threading; (d) organisation by subject/topic; (e) freedom to start new boards; and (f) a lot more intelligent debate than G+. Fixing all of these G+ deficiencies in the current system looks a long way beyond Google's capabilities, even if it wanted to.
The dog-slow operation of G+ and the extreme information-poverty of the UI (which makes it impossible to scan topics quickly) add to G+'s problems in being a pseudo-Reddit.
I was with you until you said intelligent debate. Aside from a few pockets like /r/askscience, Reddit is an awful place for an intelligent conversation. I don't know if Google+ is worse, but there are great pockets there too (Linus Tovalds for instance).
Reddit is certainly variable, but I often find the debates interesting. On G+, I mostly see people on soapboxes: it's not so much debate as the defending of entrenched positions....
"In line with Google’s vision to organize the world’s information, the focus on Google+ seems to be shifting to content more than relationships. ... I wonder if Google is more interested in being Reddit (the front page of the Internet), than it is in being a Facebook/Twitter clone (what your friends are up to)."