Tim has couple posts that lists the features of Silverlight 4 comprehensively:

Read this first to get the list as was announced during last PDC: http://timheuer.com/blog/archive/2009/11/18/whats-new-in-silverlight-4-complete-guide-new-features.aspx

Few changes to the features listed above: http://timheuer.com/blog/archive/2010/03/15/whats-new-in-silverlight-4-rc-mix10.aspx

All in all, Silverlight is evolving into a comprehensive client platform for building rich applications inside or outside the browser.

-Hanu