Main two features that seem to be quite different when using Discourse are:
Cannot reply* via email subject line to a specific topic thread as you would on Mailman.
You’ll want to update your documentation so users know to use the Watch and Mute options in email replies. Plus, definitely follow the guide linked above. ^^
Install Expose Email In plugin so users without an account can click an email icon in order to anonymously create posts on your forum in specific categories.