Interesting indeed… Not sure about all the other points (have not checked), but even if I repeat myself:

Easy-to-use single-sign-on across servers. Use case: I use several apps for different content types (like micro blog and video). Bonus: they all post from the same identifier

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OpenWebAuth in Zot (Hubzilla) and Nomad (streams)

Easy-to-use persona management. Use case: I have a personal and a work account, bonus if they can be on the same server

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Multiple identities, not only on the same server, but (if you wish so) tied to the same account (same login+credentials) from where they can be switched with a click.

Plus, in Hubzilla, each identity can have multiple profiles to show to different contact lists (This is a bit over the top, maybe, hardly anyone uses it, but you can show a different faces and profile data to different privacy circles, and post to different pricavy circles, even from the same identity)… Streams skipped that for ease of use.

Identifiers not tied to the domain name system

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In Zot & Nomad, every identity has an own address-independent ID+hash which can be cloned to multiple addresses (one of which can be defined as the currently “primary” address, but you can access everything from any clone).

Sorry, I am aware that I am naming Hubzilla and streams often currently - But it seems the functionality it offers is currently in focus, but not widely known…