The experience with mailing list mode is intended for smaller forms. Indeed it was so horrible it is now no longer available (as you would get all topics from all languages).
We have written some better instructions for those transitioning an email list to discourse here: Moving your mailing list to OSGeo Discourse
I think some better istructions for users could be supplied - here are some isolated examples:
- Welcome to GeoServer User Forum - an example of a public email list, no group required
- About the pgrouting-dev category - an example of a developer list where joining the group sets everything up