ASP.NET Ajax Improvements
ASP.NET Dynamic Data Support
ASP.NET Silverlight Support
ADO.NET Data Services
A set of ASP.NET 3.5 quickstart samples are also available.
A quick take:
Q: Should I use this for production development?
A: No, this is a CTP Preivew. See the ASP.NET Product Roadmap for more information.
Q: If I shouldn't use this for production development, what good is this release?
A: It provides the ASP.NET MVC which if your requirements need it provide the ability for enhanced URL control for your ASP.NET applications. Previously, the only ways to do a very complex URL structure was a painstaking folder path, or installing an Isapi extension.