I've been using #ActivityPub for so long that it wasn't until I spoke to an #ATProto -using friend that I realized there are a lot of issues we need to fix to make it easier for mass-adoption.
(And yes, mass adoption is important. Whatever is adopted at large becomes something of a social imposition, a necessity to survive in the world...so I'd rather we adopt truly decentralized platforms rather than faux decentralized ones.)
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Small issues:
1. A big server connecting to a small one for the first time can overload it. (Can't confirm.)
2. Searching for usernames can be slow. (Not IME)
3. Editing posts doesn't always update them on other servers.
Big problems
1. You need a separate account for each ActivityPub service.
2. Native apps can't open links directly - need to copy-paste into the search bar.
3. Apps usually don't show the real like/boost count - only the ones from your own server.
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4. Profiles of users on "new" servers can look empty until the initial fetch.
5. Server defederation can prevent legitimate users from communicating. Users should be able to override server blocks for specific users at a personal level. (This is already possible with certain kinds of blocks, but servers often use the highest-power block which doesn't let this happen.) Server blocks should be clearly communicated to users within the clients.
6. Moving servers is lossy. *stares at #Mastodon*
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