Smaller framework size. Granted SL4 will probably be much bigger than SL3, but currently the Silverlight framework is about 1/10 the size of the smallest version of .NET 3.5.

Cross platform support- Silverlight runs on Mac and Linux(in theory). This may not be a big issue to you but it is critical in some scenarios.

Much better integration with HTML. Silverlight can live inside a web page and with version 4 html can live inside of Silverlight. Once again, this may not apply to you, but if you need to interoperte with exisiting web apps, Silverlight is definitely the way to go. It will also make it much easier to transition to the web is you need to.

It's clearly where Microsoft if putting its energy. I wouldn't be surprised if WPF is pretty much dead in the water, much like Winforms and LINQ to SQL.