Threadable is like Google Groups, except that whether or not the default reply-to is "all" is a user preference rather than a group preference, which made me so happy.

(There are other features too, which I entirely ignore without missing anything. In fact, other than "there are some HTML buttons in the email", which will probably also eventually have an option to turn off, our transition from our prior mailing list service was almost unnoticeable for somebody like me who didn't want to use the extra features. And that's a compliment.)