1) Multi-Targeting Support
Earlier you were not able to working with .NET 1.1 applications directly in visual studio 2005. Now in Visual studio 2008 you are able to create, run, debug the .NET 2.0, .NET 3.0 and .NET 3.5 applications. You can also deploy .NET 2.0 applications in the machines which contains only .NET 2.0 not .NET 3.x.
2) Vertical Split View :
This feature show both design and source code in single window. To enable vertical split-view orientation in VS 2008, select the tools->options menu item and go to the HTML Designer->General section. Then check the "Split views vertically" checkbox
4) Web Design and CSS Support
Microsoft has added new Web Design and CSS Support to Visual Studio 2008
5) AJAX & Silverlight Library support for ASP.NET
Previously developer has to install AJAX control library separately that does not come from VS, but now if you install Visual Studio 2008, you can built-in AJAX control library. Also in VS 2008 Silverlight Library is inbuilt.
6) Access to .NET Framework Source Code
you can debug the source code of .NET Framework Library methods.