I assumed exposing emails and the “reply” function of email clients would be the two biggest issues.
Letting any user discover the email of any other user on the system would be a big deal for a lot of communities. I understand listservs operate that way. Could be abused in lots of different ways and require additional security/privacy features. Would have to be opt-in (user account setting; defaulted to off) for existing communities and disclosed fully for new communities. If the private messaging features work via email, not sure why a user MUST HAVE the email address. It would eliminate protections like blocking users and add potential liability to community operators (email address can expose a lot of info about people). Users can choose to put that info in their profile or post.
I agree the reply-to issue is important for users. The reply-all feature of most email clients is not the default and sort of hidden many times. So we are making community users do more work to participate in the community or, even worse, users may think they are replying to the community (current Discourse default) and it just goes to the one user.
Consider the following:
If the user hits “Reply”, then it goes to the user who sent the message [username] [firstname.lastname@example.org] AND the community [sitename] [email@example.com] (with the replies keys for each listed separately in the To: field. The user could delete the sitename address out of the To: field if they choose. (Would need to not send duplicate emails to the recipient if both emails remain). Alternatively, just add a setting so users can choose the reply functionality (like in your screenshot above). This would also fix the confusion about “reply to this email to respond.” I’d support that approach.