1) Creating a new identity and communicating it to people is a chore. Whatsapp just uses your existing email and phone number.
2) Creating a new list of contacts is a chore. Whatsapp just uses your existing contact list of emails and phone numbers.
3) At least on iOS, signing in to a service and managing session timeouts is passé. With Whatsapp you install it, run it once, and the Push Notification API handles the persistent connection and receiving messages without ever having to launch the app again. Or without even making sure it's running.
4) iMessage and BBM do the same thing, but those are nonstarters in a multi platform world. Neither service is going to go multi platform any time soon if ever.
Whatsapp is the Dropbox of messaging. Everything else just looks outdated and clunky in comparison.
That said, I'm really disappointed in Google. Google is positioning GTalk as a competitive advantage for Android. There is no reason there shouldn't be official GTalk clients on every platform. GTalk on iOS would be more popular than all of Google's other iOS apps combined.