@jwildeboer I think the long-form and comprehensive discussions you usually find on mailing lists are quite different from the focused ones that should be in issues.
Granted, you can do minute back and forths about diffs and patches in email and try to dissect strategic concerns in github issues, but I think that separating the two alone is worth doing it via email, never mind that I think the different composition environment forces a bit of switch in mind set – text boxes in web browsers are rather casual and ephemeral.
D's NNTP-backed fora might be another good approach here, but alas, who has a good, dedicated Usenet client anymore, so this just ends up as yet another web forum.
I used to think that Wikis were the next level of common strategic discussions, multiple people authoring a proper design document. But it turns out that this isn't a good fit for both collaborative endeavors and the way a lot of people prefer to communicate, i.e. it gets tiresome for most rather wiki-wiki.