1) got rid of usernames/passwords. This was a surprisingly high barrier for non tech literate people to pass, and WhatsApp made it dead simple for them to download from app store, install, and start chatting with their contacts.
2) SMS verification eliminated spam
And the timing was perfect. Data plans and smartphones were just coming into the fore, and WhatsApp popped in as an alternative to the extremely inferior MMS option, and gave people a free way to communicate across borders.
I also remember having trouble sharing contacts prior to WhatsApp, and after WhatsApp, it didn't matter what mobileOS the other person was using, I could just send a contact, and they would be able to add it to theirs. Same with location. And the picture/video sharing was way nicer early on also, albeit heavily compressed, it just worked.