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The main things are that (1) there is end to end encryption, and (2) message are only stored locally on your device. Also, because it uses a peer-to-peer model, messages can only be exchanged when both parties are online at the same time. You can also create an account anonymously. The company has a blog that you might like to take a look at, including a more detailed description of how it works. http://engineering.bittorrent.com/2014/09/17/how-does-bleep-work/
If you want to check out the alpha, here is the link for that: