Ubiquity; works on every mobile platform. (Including Nokia/Symbian, Windows Phone) So you can talk to any of your friends if they install the app.
Phone numbers are logins. Everybody with a phone already has one, so they don't have to do the whole 'create/verify an ID/password' dance.
If you have friends in other countries, this avoids roaming SMS charges. And if you are from Ireland, or Greece, lots of your friends are in other countries.
> you can talk to any of your friends if they install the app
Yes, that's exactly the catch. Why should you require them to install anoter app if they might already use some other IM network? Approach of WhatsApp may be simplifies the initial registration step, but it adds to the global mess of the non interoperable IM networks. Negative impact way outweighs any potential comfort benefits, and authors who promote such things are to be blamed for it.
> Everybody with a phone already has one, so they don't have to do the whole 'create/verify an ID/password' dance
Actually you might want to do it, since numbers change, while IDs don't. Plus you want to authenticate the other party if for example you need a secure conversation (such as with OTR).
For most people it's far easier to just install an app instead of going through a whole setup and registration process and remembering a login handle and a password. I have a lot non-geek friends who are happy with how WhatsApp works. They mostly ignore all privacy issues with this.
I'm not really directing this critique to WhatsApp users, rather to WhatsApp authors, who exploit users' comfort of no configuration vs one time configuration / registration, while causing with that a proliferation of walled networks. They are not doing a good service to users at large.
One time configuration / registration is not really a burden. All users are familial with that process. And they don't do it each time they read their e-mail for example. As I said, the negative impact of non interoperability proliferation is way more significant.
Phone numbers are logins. Everybody with a phone already has one, so they don't have to do the whole 'create/verify an ID/password' dance.
If you have friends in other countries, this avoids roaming SMS charges. And if you are from Ireland, or Greece, lots of your friends are in other countries.