What kinds of extensions should the typical activitypub user be worried about? I don't care if Meta adds payments or virtual avatars or whatever--if the core functionality of the Threads app is simple microblogging, it should be perfectly interoperable with that side of the fediverse.

The more likely effect IMO (if Meta holds to their word on enabling federation on their side) is that other large social media companies (e.g. reddit, twitter) will feel pressured to federate and that will make the fediverse better, not worse.

My account is on kbin.social but I'm working on getting kbin self hosted. When I do, I'll absolutely be federating with Threads whether or not kbin.social does.