* End-to-end encrypted by default, to the point that it's difficult to send plaintext.
* Radically simplified UX that doesn't meaningfully involve key management, which lawyers and activists cannot handle.
* By-default support for message log grooming / "disappearing messages".
* Modern cryptographic primitives.
* Extensively vetted cryptographic design.
* Works on phones, which are (1) the platform of choice for ordinary users, and (2) almost always more secure than desktop computers.
If you narrow the list down to Signal, you get to further add:
* Serverside privacy optimization / metadata minimization, so there's no targetable repository of all-pairs communicating parties, which is extremely valuable information for state-level adversaries.
* A commitment to not releasing features that don't square with those privacy objectives, so that for instance you can't share GIFs in the app until Moxie and his team figure out how to tunnel Giphy requests to foil traffic analysis, and you don't even get user profiles until a year or so ago, when Moxie and his team figure out how to provide them without generating a serverside database of identities.
* A multi-year track record of deep security auditing and a high-profile recipient of volunteer auditing unmatched by any other messenger.
