Linking to issues and pull requests on Github is as easy as typing #123, where 123 refers to the number in the URL, i.e. github.com/some/repo/issues/123 This allows for easy cross-referencing in discussions without having to copy and paste URLs around.
I know e.g. that issue 2566 in the Keystone repo is my admin interface overhaul, and issue 250 is the support feature request for nodemailer. Without that feature, I'd have to search for those often-linked-to issues every time.