"how is WhatsApp any different than iMessage / Facebook Messenger / Hangouts?"
That's not the right question to begin with.
The right question on this is: are those instant messenger users on WhatsApp worth much financially, will they ever produce financial returns, such that you justify $19 billion in any respect. I say no, the value of messenger bytes is nearly zero unless you can hold the users hostage and extract large sums from them (as AT&T and Verizon did on SMS). All user actions are not created equal in terms of the economic value they generate - one network of actions is not inherently as valuable as another; this premise will eventually burst the myth of Snapchat and WhatsApp being worth their current valuations.
How much are all those users worth? Not much if they don't actually generate cash, which is why webmail and traditional instant messaging is worthless financially.
WhatsApp was a threat to them, and that demonstrates just how financially dark Facebook's future is. Facebook will have to perpetually destroy its shareholder value buying up non-performing businesses to try to stall those upstarts from destroying Facebook's temporary ad platform. Facebook is likely to get Craigslist'd (replaced by an endless parade of dirt cheap social apps with no business prospects), because the real financial value of what it does is low.
The app itself is very useful to its users, but it's not financially worth much, no different than Gmail or Yahoo Mail (which both have hundreds of millions of users that send bazillions of messages per day).
Take away: the messages aren't worth much financially; the users aren't worth much financially. Five years from now, people are going to look back on all of this, and think it was crazy how much was being paid for "users" and "messages."