What's New in the .NET Framework 4

Garbage Collection - The .NET Framework 4 provides background garbage collection. This feature replaces concurrent garbage collection in previous versions and provides better performance. Get More information here about GC
Dynamic Language Runtime - System.Dynamic namespace is added to support this.
Covariance and Contravariance
BigInteger and Complex Numbers - System.Numerics namespace is introduced to support this.
File System Enumeration Improvements - Directory.EnumerateDirectories & Directory.EnumerateFiles can be used to loop through the Dirs & Files
Migration issues - For migration issues check the microsft site.
Parallel Computing - The .NET Framework 4 introduces a new programming model for writing multithreaded and asynchronous code that greatly simplifies the work of application and library developers.The new System.Threading.Tasks namespace and other related types support this new model.
C# New Features

• String.IsNullOrWhiteSpace
• StringBuilder.Clear
• Thread.Yield

Web

• Core services, including a new API that lets you extend caching, support for compression for session-state data, and a new application preload manager (autostart feature).
• Web Forms, including more integrated support for ASP.NET routing, enhanced support for Web standards, updated browser support, new features for data controls, and new features for view state management.
• Web Forms controls, including a new Chart control. MVC, including new helper methods for views, support for partitioned MVC applications, and asynchronous controllers.
• Dynamic Data, including support for existing Web applications, support for many-to-many relationships and inheritance, new field templates and attributes, and enhanced data filtering.
• Microsoft Ajax, including additional support for client-based Ajax applications in the Microsoft Ajax Library.
• Visual Web Developer, including improved IntelliSense for JScript, new auto-complete snippets for HTML and ASP.NET markup, and enhanced CSS compatibility.
• Deployment, including new tools for automating typical deployment tasks.
• Multi-targeting, including better filtering for features that are not available in the target version of the .NET Framework.

Windows Communication Foundation

• Configuration-based activation: Removes the requirement for having an .svc file.
• System.Web.Routing integration: Gives you more control over your service's URL by allowing the use of extensionless URLs.
• Multiple IIS site bindings support: Allows you to have multiple base addresses with the same protocol on the same Web site.
• Routing Service: Allows you to route messages based on content.
• Support for WS-Discovery: Allows you to create and search for discoverable services.
• Standard endpoints: Predefined endpoints that allow you to specify only certain properties.
• Workflow services: Integrates WCF and WF by providing activities to send and receive messages, the ability to correlate messages based on content, and a workflow service host.
• WCF REST features: Web HTTP caching, Web HTTP formats support, Web HTTP services help page, Web HTTP error handling, Web HTTP cross-domain JavaScript support

IDE

• Improved Intellisense
• Multi monitor support
• Call hierarchy for methods