Abaixo a lista dos recursos que estão disponiveis nesta versão beta do Silverlight 5:

• Multiple Window Support
• Ancestor RelativeSource Binding
• Implicit DataTemplates
• ClickCount
• Binding on Style Setter
• Realtime Sound (low‐latency Audio)
• Variable Speed Playback (“Trick Play”)
• Linked Text Containers
• Custom Markup Extensions
• XAML Binding Debugging
• 3D Graphics API
• Hardware video decode for H.264 playback.
• Multi‐core background JIT support for improved startup performance.
• ComboBox type ahead with text searching.
• Full keyboard support in full‐screen for trusted in‐browser applications, enables richer kiosk and media viewing applications in‐browser.
• Default filename in SaveFileDialog – Specify a default filename when you launch the SaveFileDialog.
• Unrestricted filesystem access – trusted applications can Read write to files in any directory on the filesystem.
• Improved Graphics stack – The graphics stack has been re‐architected to bring over improvements from WP7, such as Independent Animations.
• Performance optimizations.
• XAML Parser performance optimizations.
• Network Latency optimizations.
• Text layout performance improvements.
• Hardware acceleration is enabled in windowless mode with Internet Explorer 9.

E abaixo a lista dos recursos que foram prometidos no evento Firestarter e ainda não estão disponíveis neste beta do Silverlight 5:

• Support for Postscript vector printing enables users to create reports and documents, including
  the ability to create a virtual print view different from what is shown on the screen.
• Improved power awareness prevents the screen saver from being shown while watching video
  and allows the computer to sleep when video is not active.
• Remote control support, allowing users to control media playback
• DRM advancements that allow seamless switching between DRM media sources.
• Enhanced OpenType support.
• Fluid user interface enables smoother animation within the UI. Inter‐Layout Transitions allow developers to specify animations to apply when elements are added, removed or re‐ordered within a layout. This provides smoother user experiences when, for example, items are inserted into a list.
• Text clarity is improved with Pixel Snapping.
• The DataContextChanged event is being introduced.
• WS‐Trust support: Security Assertion Markup Language authentication token.
• 64 bit support
• COM Interop for trusted applications running in‐browser.
• Calling unmanaged code using P/Invoke from trusted applications in and out of browser.
• The Pivot viewer control enables a visual way to rapidly sort through large collections of graphical data, for example a catalog of movies represented by images of dvd covers, using intuitive transitions to show the effects of filters on the result set.

Como a maioria dos termos são em inglês, não traduzi o conteúdo da documentação da Microsoft que pode ser acessada aqui.