A good friend of mine and I chat online nearly every day. We use both our smartphones and computers, so we wanted an app with which we could seamlessly carry on our conversation(s). For a long time, we've been using Skype. We've been able to chat via text or video, and exchange pictures/files. Recently though, we started not receiving each other's messages. This was huge for us, since we get concerned if we don't hear from one another. This went on for a while, and we finally decided it was time for a change. That's when we found WhatsApp.
I wasn't super thrilled that it's owned by Facebook, but we thought we'd give it a try. Both of us love it! We can use it on both our smartphones and computers just like we want, and we can carry on text or video conversations. We can also transfer files just as easily as we could before. The benefit to switching wasn't just limited to ensuring our messages were received though. Through the smartphone app, the conversation background can be changed to a photo of the user's choosing, the notification sound may be customized, and new messages may show up on the lock screen and even be replied to without having to unlock the phone - none of those features were offered with Skype.
Most messaging apps are all pretty much the same. You make sure your contact has the same app, you start a conversation, type, send, and that's it. But it's the little things that make a difference. Even ignoring the fact that Skype wasn't sending/receiving messages like it should, the "bonuses" of WhatsApp make it an even more pleasant experience. I recognize that simple features can make or break an app.