Fediverse

I think the killer feature for most people was simply that anyone whose number you had in your phone's address book would, without any further configuration, become available as a WhatsApp contact if they, too, had installed the app. That made it usable even by people so unfamiliar with technology that the concept of creating an account was foreign to them. No password, no username, just your phone's address book and the app, that's it. Open IM technologies at the time like XMPP didn't have this (at least to my knowledge), and even today, while Matrix has this feature via identity servers, it takes manual effort to set it up (as it should be, though it could be streamlined somewhat in Element AFAICT).