Like our Trac, it's tied into GitHub authentication, so there's no need to register another account or anything.
While this is still technically a test period, we'll likely make it fully official within the next month or two.
There are sub-forums/categories for both Django Internals (the things this list talks about) and also Using Django (django-users content) - and if you feel like helping people out, you can watch an entire category inside of Discourse (click the circle on the top-right of a category page) and it'll email you about everything that gets posted. You can even reply to those emails, or sign up for a daily digest, if you want to keep the mailing list feel.
We're not deprecating the mailing lists or anything - the forum is meant to supplement them, and especially to provide a more accessible place for new Django users and developers to ask questions without the perceived overhead/global broadcast of a mailing list. It'll also provide us with a nice way to have a lot of async discussions (https://forum.djangoproject.com/c/internals/async) without bombarding the rest of you!