Overview

The .NET Framework is Microsoft's comprehensive and consistent programming model for building applications that have visually stunning user experiences, seamless and secure communication, and the ability to model a range of business processes.

The .NET Framework 4 works side by side with older Framework versions. Applications that are based on earlier versions of the Framework will continue to run on the version targeted by default.

    The Microsoft .NET Framework 4 provides the following new features and improvements:
    • Improvements in CLR and BCL
      • Performance improvement including better multicore support, background garbage collection, and profiler attach on server.
      • New memory mapped file and numeric types
      • Easier debugging including dump debugging, Watson minidumps, mixed mode debugging for 64 bit and code contracts.
    • Innovations in the Visual Basic and C# languages, for example statement lambdas, implicit line continuations, dynamic dispatch, and named/optional parameters.
    • Improvements in ADO.NET
      • Entity Framework, which simplifies how developers program against relational databases by raising the level of abstraction. The Entity Framework has many new features in the .NET 4 Framework. These features include persistence ignorance and POCO support, Foreign Key Associations, lazy loading, test-driven development support, functions in model, and new LINQ operators.
      • Data Services framework consists of a combination of patterns and libraries that enable the consumption of REST-based data services for the web. ADO.NET Data Services has many new features in the .NET 4 Framework. These features include: enhanced BLOB support, Data Binding, Row Count, Feed Customization, Projections, and Request Pipeline improvements. Built-in integration in Microsoft Office 2010 now makes it possible to expose Microsoft Office SharePoint Server data as a data service and access that data using the ADO.NET Data Services client library.
    • Enhancements to ASP.NET
      • More control over HTML, element IDs and custom CSS that make it much easier to create standards-compliant and SEO-friendly web forms.
      • New dynamic data features including new query filters, entity templates, richer support for Entity Framework 4, and validation and templating features that can be easily applied to existing web forms.
      • Web forms support for new AJAX library improvements including built-in support for content delivery networks (CDNs).
      • For a comprehensive list of enhancements to ASP.NET go here.
    • Improvements in WPF
      • Added support for Windows 7 multi-touch, ribbon controls, and taskbar extensibility features.
      • Added support for Surface 2.0 SDK.
      • New line-of-business controls including charting control, smart edit, data grid, and others that improve the experience for developers who build data centric applications.
      • Improvements in performance and scalability.
      • Visual improvements in text clarity, layout pixel snapping, localization, and interoperability.
    • Improvements to Windows Workflow (WF) that enable developers to better host and interact with workflows. These include an improved activity programming model, an improved designer experience, a new flowchart modeling style, an expanded activity palette, workflow-rules integration, and new message correlation features. The .NET Framework 4 also offers significant performance gains for WF-based workflows.
    • Improvements to Windows Communication Foundation (WCF) such as support for WCF Workflow Services enabling workflow programs with messaging activities, correlation support. Additionally, .NET Framework 4 provides new WCF features such as service discovery, routing service, REST support, diagnostics, and performance.
    • Innovative new parallel-programming features such as parallel loop support, Task Parallel Library (TPL), Parallel LINQ (PLINQ), and coordination data structures which let developers harness the power of multi-core processors.

For more details

 http://msdn.microsoft.com/en-us/vstudio/dd582936.aspx