The above situation occurs because:

1) When you run Visual Studio 2010 the first time on a machine that has had Visual Studio 2008 installed on it, it asks you if you want to import your existing VS 2008 profile settings.  By default this checkbox is checked – which means you by default automatically import your existing settings.

2) Some VS plugins – for example Resharper – turn off the built-in C# code intellisense within VS and instead replace it with their own implementation.  If you’ve installed Resharper on VS 2008, the above VS code intellisense profile settings are turned off.  When you import your existing profile during VS 2010’s first time run experience the code intellisense settings import over in a disabled state.  If you haven’t installed Resharper on VS 2010 (which is a separate install) – then by default you’ll end up with intellisense turned off.