I presume this is here partly because Threads is in the news. To me interesting part will be what subset Threads will support?

For instance, ActivityPub specs start like this:

"It provides a client-to-server API for creating, updating and deleting content, as well as a federated server-to-server API for delivering notifications and content."

However, that client-to-server API is not even implemented by Mastodon. It's unlikely Threads will support it either, leaving those who want to make own clients in the cold.

Then there is stuff like outbox, which contains past messages, but for instance, Mastodon does not fetch old messages at all, causing a subpar experience: "Older posts from other servers are not displayed" when browsing profiles.

If Threads don't see the need to support outbox either, it makes browsing Threads content from your own small server an infuriating experience.