I was thinking roughly the same thing: typically when I'm researching a topic for the first time I will come across forums in search results which I will read before wanting to post. (As codinghorror has said, it's generally better etiquette to read existing forum content before posting.) I don't like creating accounts unnecessarily, and most forums* only require users to be logged in for posting (not for reading). If I did this on a Discourse forum, I would lose my 'reading credit'.
With your single sign-in support, creating an account of course becomes far less onerous - but it's still not necessarily something I'd want to do immediately on first visiting a forum.
Given the client-side cleverness of Discourse, would it be possible to track 'user reading' of a forum before the user created an account and apply its 'credits' to the account on its creation? This wouldn't solve chronodekar's problem, but it would solve mine.
*I have come across forums which hide links or images for users that aren't logged in. Personally, I find this intensely irritating and tend to look for other forums!