One big difference with Hushmail is that they are a web app, so they can be (and have been) compelled by law enforcement to serve malicious code to certain user

One big difference with Hushmail is that they are a web app, so they can be (and have been) compelled by law enforcement to serve malicious code to certain users. Parley is a standalone desktop/mobile app, so it would be difficult for that scenario to occur. The passphrase is also PBKDF2'd on the client, so the server never sees it.