The key success of Whatsapp is IMHO that it uses your phone's address book as contact list. There is no hurdle to use it what is very important for the average user.
The multi platform support from beginning is second thing. J2ME as stated in the article is negligible. I can't understand why this article is on top of HN.
The phone number as an id is a very important component to their adoption indeed. So do you posit that WhatsApp's adoption rates being what they are (compared to say Viber which does the same thing) is purely due to first mover?
