Whatsapp is limited to smartphones, whereas gTalk, AIM, Yahoo messenger, MSN etc.. are true cross platform messenger services. They are available on any phone, PC, Mac, Linux box, gaming consoles, etc..
I'm personally boycotting Whatsapp, PingChat, Kik, KakaoTalk etc... simply because I don't feel the desire to install yet more messaging apps (or ask others to install new messengers on their devices) when there are already plenty of messengers available. I already have accounts with the 5 major IM services, and when I meet somebody new, chances are good they already have an account with one of them.
IMHO, when there are already plenty of all-in-one messenger apps like Trillian, IM+, etc.. each of which already support 9 or 10 services (I use Trillian myself), there's no need to reinvent the wheel.