It would require ActivityPub to be prescriptive about how you're supposed to do certain elements of your application, if you choose to do those parts. This is Markdown, this is how you write a link to something in your instance, this is how you handle a threaded community, this is what disliking a message looks like. From my outsider's perspective it seems like it wouldn't have been that challenging to lay down guidelines for it.

Eh, whatever. I have my gripes but they're working on it and it seems okay. It just seems that each application is a sort of little walled garden that works (mostly) with other instances of the same app, but for anything other than that same app all bets are off, which seems like a shame.