Silverlight uses MANAGED CODE.
That means you can use .Net languages -- particularly C# -- to dev with. (You can also use JavaScript, which many do, but your source code is visible to anyone using your application.) And with C#, you get a COMPILED application which (usually) runs much faster than a scripted language like ActionScript.
Also, Silverlight uses XAML, an XML-based language that allows designers and developers to work independently. Your designer can worry about how the application looks while the developer can worry about how it works. This is an essential aspect of the .Net paradigm.