Back in the day, I remember using AIM for group chat with friends and instant messaging. Although I've never used Whatsapp, it seems like a mobile equivalent of AIM.
There are some differences which is possibly because it's the mobile equivalent.
One is that you don't have to add any new contact. If anyone on your phone book has whatsapp, they instantly show up i.e your identity is your phone number.
Two when you join a group if there is a person already in the group whose you know in real life, but don't have the number of, it's easy to get their contact into your phone book.
I was able to reconnect with quite a few of my old high school friends using whatsapp. The interesting thing was that everyone spread around the world. Many are in the US, but several are in UK, some are in the middle east, many in India, a couple in Singapore and the group chat made it possible for everyone to instantly connect without having to "add a friend". Plus, no cost difference to send a message to someone in the UK or India.
Like you said, it's the mobile equivalent but the group chat feature is what makes it really useful esp. if you have friends in multiple countries.