If you want know all about Silverlight, there's loads of info online.
What remember is that it's cross browser/cross platform from day one. Proof is that I installed the plugin for Firefox on my Mac during the speech and it works seamlessly. Although I must say the install process is a bit too complicated (download, extract, install, restart browser).
Second is the attention they give to video. Extremely high quality streaming (up to HD), watching video sync-ed with friends over the web and chatting about it, uploading user generated movies (out of the box), etc. Looks like a strong competitor for Flash media server.
Go download the plugin (beta):
Check out some demo's (although they do not seem to work, maybe due to low bandwidth in the room here):
http://msdn2.microsoft.com/en-us/default.aspx (works, but not really spectacular)
http://silverlight.net/fox/ (THIS ONE YOU WANT TO SEE)
http://silverlight.net/showcase/Default.aspx (doesn't work)
http://ds.pilot.serving-sys.com/BurstingRes/WPFE/msn_tech.html (doesn't work)