I am a BlackBerry user and while I am not expert on WhatsApp or Viber, here are the main features of BBM on my Q10/Z10 (I have both).
* Standard chat
* Video Chat
* Voice Only Chat
* Screen sharing - I can share my Z10/Q10 screen out with whoever I am BBMing with. A great way to share photos, documents or even just have someone watch me run an app. I have used it a few times in a business scenario and it works quite well
* Send files, videos, docs, etc
* BBM Channels - This features is still in Beta so not sure you will see it on iOS/Android for a while. In a nutshell, it adds a social networking aspect to BBM. I can update my 'channel' and all those who follow can see my updates. There are some other aspects to this that makes it more interactive (ie. Followers can initiate a chat with the channel owner) but thats the gist of it.
* Add contacts using NFC - Wont be possible on iOs since none of the phones have NFC (though I am sure its just a matter of time)
* Chat Groups - To be honest this is not a features I have used before so but I do see the option is available on my device.
Not sure if I have missed anything but I believe thats the crux of it.
One other thing is, I have heard that BlackBerry are trialling a service called BBM Money in Indonesia. While I have not used it, my understanding is that it enables BBM to transfer money between users. I am not saying this is a feature that will ever launch on iOS/Android but you can see now that BlackBerry are looking way beyond chat/video chat for BBM... the possibilities are intriguing and I think its great that you guys will probably have access to these features too.
Regards,
Alon