I use WhatsApp almost daily. I wanted to do a redesign of it firstly because I realized I enjoy designing (iPhone for now) apps and secondly cause I grew tired of how it looks and works now. I tried to make it more user friendly and visually appealing. Instead of the 5 tabs in the tab bar, I found 3 tabs to be sufficient: Chats, Write New and Contacts. Hence the Favorites bar went with Contacts, Status in the Chats screen (somewhat like the Yahoo Messenger app for the iPhone) and Settings almost all across the screens, in the Navigation Bar, upper right corner.
I'm thinking of redoing the icon as well as soon as I get my head around the fact that you can't call for free with WhatsApp (like with Viber) but they have a phone in their logo.
Update: this little guy got featured on redsgned.com! Here's the link: http://redsgned.com/whatsapp-for-iphone-redesign/