Jason Zander's blog entry on the Silverlight announcement contains a very important information nugget:
2. The APIs we are releasing are "right sized". That means we looked for the most powerful subset we could find while keeping the size small.
This I think is what makes Silverlight so interesting to me. Compared to WPF which is a complete framework for desktop applications, they are doing an effort to keep the APIs to a minimum.
Default Widgets: Silverlight comes with no pre-defined widgets, and although at first this seems like a weakness, during one of the presentations today it was clear that one-size-fits-all was not going to give everyone what they wanted.
I believe it was during Mike Harsh presentation, where he created a nice looking custom button out of simple primitives.