1.) New Web Design Interface
Visual Studio 2008 has incorporated a new Web designer that uses the design engine from Expression Web. Moving between design and source view is faster than ever and the new split view capability means you can edit the HTML source and simultaneously see the results on the page. Support for style sheets in separate files has been added as well as a CSS properties pane which clarifies the sometimes-complex hierarchy of cascading styles, so that it is easy to understand why an element looks the way it does. In addition Visual Studio 2008 has full WYSIWYG support for building and using ASP.NET Nested Master Pages which greatly improves the ability to build a Web site with a consistent look and feel.
2.) JavaScript Debugging and Intellisense
In Visual Studio 2008, client-side JavaScript has now become a first-class citizen in regards to its debugging and Intellisense support. Not only does the Intellisense give standard JavaScript keyword support, but it will automatically infer variable types and provide method, property and event support from any number of included script files. Similarly, the JavaScript debugging support now allows for the deep Watch and Locals support in JavaScript that you are accustomed to having in other languages in Visual Studio. And despite the dynamic nature of a lot of JavaScript, you will always be able to visualize and step into the JavaScript code, no matter where it is generated from. This is especially convenient when building ASP.NET AJAX applications.
3.) Multi-targeting Support
In previous versions of Visual Studio, you could only build projects that targeted a single version of the .NET Framework. With Visual Studio 2008, we have introduced the concept of Multi-targeting. Through a simple drop-down, you can decide if you want a project to target .NET Framework 2.0, 3.0 or 3.5. The builds, the Intellisense, the toolbox, etc. will all adjust to the feature set of the specific version of the .NET Framework which you choose. This allows you to take advantage of the new features in Visual Studio 2008, like the Web design interface, and the improved JavaScript support, and still build your projects for their current runtime version.