Make it so that any windows user wouldn't know they weren't on Windows other than the missing Windows icon in the start button. Integrate Wine so it is seemless.
Make it so if I know how to use Windows, I can know how to use a Linux desktop.
There will need to be a classic mode for those like me who prefer the clean classic Windows 98 desktop, as well as one for those that like the Aero Glass or whatever that pretty mode is. (I don't like pretty mode, when I want to see the window behind my primary, I'll give it focus! I don't need to watch my window minimize, just get it out of my way!)
If you want to switch a desktop user, make it so the user doesn't have to know they switched.
IMarv