The biggest thing to note about the new messenger, by far, is that the developer intends to provide end-to-end encryption. More impressively, the app is intended to work with any Fediverse account.

This would make cross communication across the fediverse easier and may even interest others to join. It gives a leg up to the InstaRedFaceXTwitThreadApps since you can just use your one fediverse account.

There are definitely some open questions about how Sup works with the rest of the Fediverse. Do users just sign in to a service via an OAuth dance with their Fediverse accounts? Do the messages federate over ActivityPub? Does my server store my messages, or does just the app store those?

These questions and more would have to be answered before I signed up though.