In terms of displaying content, asking questions is a very different thing to discussion. If you've ever had to trawl through forum threads trying to find answers, it is a nightmare. StackOverflow is much more heavily moderated and curated to keep the answers from being buried.
In general, you aren't going to be able to discuss something on SO, and people aren't going to find answers in discussion boards because they don't make it easy to find those answers.
I know what you mean, but given great search, this is not going to be massive difference.
But the ability to link to source code lines/issues/pull-requests as part of the discussion is going to be good.
Here's the thing - I would bet there's an overwhelming majority of users, who "discuss" and ask on GitHub issues anyway. And half the job of repo owners is to close such issues.
So it's not like the natural behaviour of users is to go to stackoverflow for questions/discussions anyway. This just channels existing behaviour
In general, you aren't going to be able to discuss something on SO, and people aren't going to find answers in discussion boards because they don't make it easy to find those answers.