Protocols and privacy and chat features are all great, but let's not lose sight of the two absolutely killer essential features of every successful chat app / system:
(1) The address book.
(2) The easiest possible signup and setup.
More generally for #1, whatever method(s) you use to find people's contact information and connect with them.
Without that, you could have created the perfect chat experience in every way, but people still won't use it. Because their friends aren't on it and/or they can't find their friends.
A ton of chat apps fail because engineers sit around talking about how to architect things, when the truth is that once you've gotten past the almost-insurmountable hurdle of getting two users onto the system and starting a conversation, the actual mechanism for actually sending messages back and forth is not much more than an implementation detail.
Not that those things aren't important, but they are like 10% of the formula for success, and the other 90% is getting to the point where they can start a chat session.
People's primary need is to communicate. With the people they want or need to communicate with. And via the contact information they have available. (Exchanging contact info for a new service offline is a huge hurdle.) A better experience is better, but at the end of the day, these other concerns will nearly always win out. So you don't get very far trying to use a superior experience as leverage to get people using it. You need to attack that problem more directly.