This might seem weird but if you use ReSharper then you might not be able to use intellisense on the QuickWatch window i.e intellisense menu doesn't automatically show up in the QuickWatch window . This is because Resharper disables this feature.
You can solve this issue by manually making the intellisense menu appear by after typing at least 1 character in the Name field of the QuickWatch window and then pressing Ctrl+Space.
C#, Silverlight, Resharper
19. October 2011