A quick summary of the key new features in ASP.NET 4:

• jQuery Included with Web Forms and MVC – probably the biggest surprise in ASP.NET 4 is the full jQuery support, including compatibility with ASP.NET AJAX
• ASP.NET MVC – now a fully-supported ASP.NET feature for improved testability of web applications
• Dynamic Data – simplifies common data entry and validation operations at the presentation layer
• Extensible Output Caching – more control over cache fragments and the implementation of the caching
• Shrinking Session State – store session data compressed for better performance
• Object Caching and Object Caching Extensibility – the ASP.NET Cache abstracted and available for non-web situations
• Content Delivery Network Support – serve jQuery libraries from Microsoft’s geo-distributed CDN
• Setting Meta Tags with the Page.MetaKeywords and Page.MetaDescription Properties – finally, no more messing about with manually adding meta tags!
• Enabling View State for Individual Controls – this could potentially increase page load speed quite substantially (although enabling GZip HTTP compression has a much bigger effect!)
• Improved Routing – no more query strings, just SEO-friendly RESTful URLs
• Inbuilt Charting – some pretty powerful (and pretty) charting features are now native to ASP.NET:

There are also a whole bunch of improvements to various controls and control renderings, such as the Wizard control, along with changes to Visual Studio, both project structure and the UI.

More info here: