I agree, this is the real reason WhatsApp took off so quickly. Especially that it automatically uses your phone number as account-id and then goes through your address book to automatically populate your WhatsApp contact list.
No other Instant Messenger did that. It was always: Create account, exchange account-ids, add to contact list. This adds huge amounts of friction for a new service, and WhatsApp sidestepped this completely.
I use KakaoTalk occasionally and KakaoTalk also does the same thing, use phone # as acct, auto populate contact using address book and also suggests friends that you might want to add.
No other Instant Messenger did that. It was always: Create account, exchange account-ids, add to contact list. This adds huge amounts of friction for a new service, and WhatsApp sidestepped this completely.