Good that you are revitalizing the discussion and I totally agree with your point about “unique usernames”. I am part of three different forums and have other nicks on all of them for good reasons, if they were forcing me to use a unified nick this would be rather painful. I actually think one of discourse strength is the decentralisation caused by allowing everyone to run their own instance.
I am currently thinking whether I want to share many of my settings throughout these forums either. Yes, the profile picture is the same but background picture and whether especially notification settings are different on purpose. Different discourse, different community, different lightyear 
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However, I do see the benefit of allowing an easy pull-my-settings-setup feature. Which could be a key point for people to join other communities, too. I am just a little concerned about keying it to the email-address. How would you make sure the recipient is trusted to receive the data? With Gravatar this is of little matter as this is only public profile-data. Which is fine for anyone with access to your email to grab. But for settings and potentially private information I am not sure that is a good idea.
Though I was thinking, it would be pretty damn sweet if discourse itself would offer OAuth-Provider functionality to allow others to authenticate against it. Essentially allowing that any discourse instance could act as a authentication provider like Twitter or Facebook. The admin could then white-list instances they consider trustworthy (meta.discourse.org being on it per default of course). Through the OAuth-Channel, a user would create the trusted channel between the two entities (both discourse instances in this case) to sync data in a federated fashion without even requiring a central hub instance nor the email-addresses to match up (which would also fail for me).
Though I’d ship the plugin per default with meta.discourse.org activated allowing for easy login and syncing against at least that instance.
A little off-topic, but becoming an OAuth-Provider would also allow better usage/extension through external bots interaction as “people” on the platform or offer other OAuth-enabled apps/features to authenticate against a discourse instance. That’d be amazing.