Just fun and interesting stuff...
I’ve been developing .NET business apps for 8 years now (since beta 1) and have found Silverlight very accessible. Keep in mind I do have WPF experience. But when I want to write some code in SL, not only do I know exactly what classes to look for, but I also bring with me the patterns I’ve used to develop with that specific set of classes for a long time now. The Flash people, of course, bring plenty of experience making animated images of monkeys swinging on trees ;)