The best I could thing of is a data driven application that has some of fancy images and shows some movies in places winform wouldn't allow them.
Like, maybe some kind of small movie database that shows the movie trailer in background while you set some values in the foreground (how many times watched, personal rating and comment, etc.).
But I don't think that's all WPF can do nor all people usually use it for (or for that matter Silverlight if it replaced WPF now).