Javascript Intellisense Support
What you may not realize if you open up a converted VS 2008 project and get no intellisense is that the designer utilizes sort of a namespace inheritance approach. For instance, if you create a new AJAX client behavior javascript file, you will see this at the top:
/// <reference name="MicrosoftAjax.js"/>
This lets VS know to use the objects defined in this file, similar to how you use "using System.Web.UI.WebControls". There are a variety of options available, and if you look through the AJAX Control Toolkit code, you will find some different files you can import into your scripts.