Thx @Jeff Chandler for including Speed Up group on this list!

As for the following I agree with you:
"I can’t help but wonder how awesome it would be if a centralized discussion platform such as a forum, played host to all of these discussions."
However, today I got idea on one of the AWP post, to create unique centralized place for all of these great WP resources/groups on Facebook, so after a few hours of data inputting here is the result - Quality WP Groups on FB:
https://www.facebook.com/groups/qualitywpgroups/

Collection of 40+ quality WP Groups on one place. :-)