UPDATE:
something happened. Google+ is now super cozy with WordPress. The Sep. 2013 update includes the functionality described below, and lets folks comment using Google+ authentication. Hurray!
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Short answer:
“At present Google+ API (released in September 2011) provides read-only access to public data, hence posts only go one way.”
If you read the reviews for the hacky WP extension mentioned there, you discover that it’s totally broken. In the words of a reviewer as of April 2013,
“This worked one time, several months ago, and since then has failed every time. This may not be the fault of the developer, but of Google, who closed the option to post that was exploited by this plugin (not positive that’s the case, but it DID work one time for me).”
Wuh?
There are tons of elegant plugins, with full API support for pushing WP content to your Facebook profile. Isn’t this a barrier to Google+ adoption, or at least a drag on its traction? Or is the Google juggernaut powerful enough to get people to post there natively/manually?