I'd like to see the HTML that is supported in e-mails standardised, so there's no more issues about which clients will render what. I'd like to see the failure notices standardised, so that invalid e-mail addresses produce a standard "404-like" response, and so on.
The problem isn't so much that we have multiple implementations (I agree that can potentially be a strength). The problem is that there's very little that's consistent across the platforms.