I saw that plugin but thought it still had the issue mentioned above.
Namely that once the user clicks the link to the forum topic for the blog post, they will need to click Log In on Discourse to be logged in on Discourse even when they are already logged in to WordPress.
SSO is good but automatic SSO where all installed applications on a web site all automatically login when the user has logged in the SSO provider would be great.
It makes things more seemless for a user to be able to login to a web site and be logged in on all the web applications used on the site and be able to hit a logout button that logs them out from all the applications.
In my case, WordPress as an SSO Provider is more of an example, I like the idea of having a site wide authentication system that is used automatically by WordPress, Discourse and other web applications that might be installed like MediaWiki.