First of all I redesigned the main window where you start selecting your contacts to chat with. I think the Skype Contacts List should look like a native Finder window, because people are used to it. The new Skype 5 for Mac has a coverflow option for browsing, but who seriously does that?
So I came up with this solution, where you can instantly start a video, a call or a normal chat. You can sort your contacts like you sort other data on your Mac.
Another big thing I would recommend Skype is to have a Responsive Application. That means, if you resize the window, you should get more or less options in the top bar (like in Apple Mail)