Early on in my career I was mentored on deferring execution. This was in a language called FORTH and the idea was build your program structure and abstract the details into WORDS to be filled in later. Repeat until complete. This simple process still holds true today in OO languages like C# where I do most of my work.
Now back to why I like Silverlight/WPF, as I'm just starting to get beyond the basics, the more I'm starting to see that this is an extremely well thought out architecture. As I'm developing my functionality I can easily "defer execution" or really in this case, care zero about the style and then go in later and make it pretty. Or if I'm really lucky find someone that knows what they are doing to give it a polished look. Although the same can be done with HTML and CSS, this just seems like it's just a bit cleaner and since we are targetting only one type of client (Silverlight or WPF) instead of the different browsers the results are much more repeatable.
The other thing I'm really impressed with is the separation of UI and code behind. Although time will tell on the actual business value (read ability to maintain and extend) it seems like the ability to create CLR instances in the XAML and then glue everything together with dependency properties, just feels good to do.
Now back to getting some work done with this!