Silverlight 3 includes a ton of new application development features.

The Silverlight 3 runtime/SDK combined with the Silverlight Toolkit now includes ~100 UI controls that enable common scenarios (layout, data, charting, child windows, etc) while also providing full styling and template customization support.

Silverlight 3 enables richer data binding features.  Element to element binding support between controls is now enabled.  Validation error template support has been added to controls (enabling better error message display).  Hierarchical data binding is supported by the DataGrid.  And a new DataForm control enables better master/detail scenarios.  Silverlight 3 also now enables SaveFileDialog support.

Silverlight 3 includes a new navigation framework that enables deep-linking and forward/back button integration within the browser.  This also enables search engine optimization (SEO) support so that content within a Silverlight application can be indexed by search engines – including Google, Bing and Yahoo.  Silverlight 3 also supports the ability to cache assemblies on the client, and re-use them across multiple applications (decreasing the download size and improving the startup time of applications).

Silverlight 3 includes much better text rendering and font support.  Text rendered using Silverlight 3 applications is much crisper and cleaner than previous releases, and applications now have access to local fonts.  The Silverlight 3 styling system also now supports merged resource dictionaries, BasedOn style inheritance support, and the ability for styles to be reset any number of times.  Silverlight 3 also adds richer accessibility support, and is the first browser plug-in to provide access to all system colors, allowing partially-sighted people to make changes such as high contrast color schemes for ease of readability using familiar operating system controls.

Silverlight 3 includes richer networking support.  WCF error faults are now supported across the network.  Server-side push duplex support is now easier to setup.  Binary XML serialization of payloads is now supported.  The new .NET RIA Services framework (which now has a go-live license) can be used to easily build multi-tier data applications that span the client and server.  .NET RIA Services enables you to write validation code once and have it applied on both the client and middle-tier layers of your applications.