It is recommended to read first 'What's New in C# 3.0'
- Support for enabling existing ASP.NET 2.0 pages for AJAX
- Creation of ASMX & WCF based web services and consuming them from AJAX Library
- ASP.NET server side application services like authentication, roles management exposed as web services
- ASP.NET Merge Tool - a new tool for merging pre-compiled assemblies
- New ListView control which supports edit, insert, delete, sorting & paging
- ASP.NET integrated with core IIS 7.0 which makes ASP.NET services like authentication & caching available for other content types also.
- Microsoft AJAX Library to support AJAX based web development
Base Classes & CLR
- Support for CSharp 3.0 including LINQ.
- HashSet : A high performance collection of type set.
- Time Zone Improvements : to develop applications which operate in multiple time zones
- Minor Improvements in Threading, Reflection and controlling the GC behavior
- Client Application Services : All windows based application to use ASP.NET login, roles and profile services hosted on central server.
- Occasionally Connected Services (OCS) : keeps windows application's local and remote data in sync.
- 'Silver' : code name for a new technology which unifies WF and WCF
- WCF Syndication : WCF services exposing syndication feeds. Supports Atom 1.0 and RSS 2.0