Any unified system probably needs to at least be capable of supporting that

It also has to have clear advantages over the default of just having a browser with multiple tabs open.

The auth problem as I see it boils down to "how can user X with an account on Less Wrong post to e.g. SSC without needing to create a separate account, while still giving SSC's owner the capability to reliably moderate or ban them."

That's an old problem. Google and Facebook would love to see their accounts be used to solve this problem and they provide tools for that (please ignore the small matter of signing with blood at the end of this long document which mentions eternity and souls...). There is OpenID which, as far as I know, never got sufficiently popular. Disqus is another way of solving the same problem.

I think this problem is hard.

most extant web forum, blogging, etc software is terrible for discussions of any nontrivial size.

That's a rather strong statement which smells of the nirvana fallacy and doesn't seem to be shared by most.