a) always end to end encrypted (unlike Telegram)
b) supports account creation without any phone number (unlike Signal)
c) supports phones without google messaging service (unlike Signal, it formally supports it as well but in reality it's just a big battery drain)
It not being open source was always a bit sad for me, because most of the other apps on my phone are open source.
I'm curious about the multi device sync. I hope that it will be implemented in a way that doesn't use passwords but pair syncing. This is bad from a usability perspective (firefox abandoned it because of usability), but the only way I'm aware of how syncing can be made secure.