• Output cache extensibility
• Session state compression
• View state mode for individual control
• Page.MetaKeyword and Page.MetaDescription properties
• Response.RedirectPermanent method
• Routing in ASP.NET
• Increase the URL character length
• New syntax for Html Encode
• Predictable Client IDs
• Web.config file refactoring
• Auto-Start ASP.NET applications
• Improvements on Microsoft Ajax Library

I’ll describe the details of each of these features in the following sections.

In ASP.NET 4.0, each web control will include a ViewStateMode property which lets developers disable view-state by default, and enable it just for the controls for which a persistence of state is required. ViewStateMode has the following three values;

• Enabled – enables the view state for this control and any child control.
• Disabled – disable the view state.
• Inherits – this specify the control uses the settings from its parent control.

• Page.MetaDescription – equivalent to meta name “description”
• Page.MetaKeywords – equivalent to meta name “keywords”

This will enable developers to easily and programmatically add the relevant keywords and description.

This will even be useful for Master pages—where you only have to add these properties in the master page. In addition to “keywords” and “description” settings in the code behind, developers can also set these values within the @Page directive.This will even be useful for Master pages—where you only have to add these properties in the master page. In addition to “keywords” and “description” settings in the code behind, developers can also set these values within the @Page directive.

Response.RedirectPermanent Method

ASP.NET 4.0 has improved SEO (Search-engine Optimization) facilities. Typically developers useResponse.Redirect(string url) to handle requests for old URLs. However, this leads to an extra round trip to access the old URLs and so will negatively affect your page-ranking  in search-engines.

ASP.NET 4.0 introduces a new Response.RedirectPermanent(string url) helper method to be used as HTTP 301 (Moved permanently) to handle requests. This will enable search-engines to index URLs and content efficiently and thus improve the page rankings.
 

Routing in ASP.NET

Routing will let developers serve meaningful URLs to users and map them with the actual physical files. This URL-rewriting mechanism enables developers to write high ranking, search-engine optimized web applications. For example, URL for a page which displays an actual product might look like the following;

By using routing the URL will look like the following

In this way, the URLs will be more easily remembered by users.  It will also significantly improve the search-engine page rankings of the web site.

The following example shows how to implement routing behavior in ASP.NET 4 using new MapPageRoute in Route class.

Increase the URL character length

In previous versions of ASP.NET,  URLs were limited to 260 characters in length. But in ASP.NET 4.0 developers have the option of increasing or decreasing  the length of URLs by using  the new maxRequestPathLength and maxQueryStringLength. I’ll illustrate this in an example.

In previous versions of ASP.NET you were limited to a fixed set of characters but in v4, developers can also validate the invalid characters by specifying values in the requestPathInvalidChars attribute.

 

More details at: http://www.simple-talk.com/dotnet/asp.net/asp.net-4.0-features/