• New language features in C# 3.0 and VB.NET 9.0 compiler
• Adds support for expression trees and lambda methods
• Extension methods
• Expression trees to represent high-level source code at runtime.
• Anonymous types with static type inference
• Language Integrated Query (LINQ) along with its various providers
  • LINQ to Objects
  • LINQ to XML
  • LINQ to SQL
• Paging support for ADO.NET
• ADO.NET synchronization API to synchronize local caches and server side datastores
• Asynchronous network I/O API .
• Peer-to-peer networking stack, including a managed PNRP resolver
• Managed wrappers for Windows Management Instrumentation and Active Directory APIs
• Enhanced WCF and WF runtimes, which let WCF work with POX and JSON data, and also expose WF workflows as WCF services. WCF services can be made stateful using the WF persistence model.
• Support for HTTP pipelining and syndication feeds.
• ASP.NET AJAX is included
• New System.CodeDom namespace.

Source: Wikipedia