I suspect that the prospect of an upgrade to Windows 8 would be the change that pushes my wife from her current Windows laptop to an Apple. Windows 7 is a lovely stable OS. There are some odd quirks about it but on the whole I don't mind using it.Well not too much.
Windows 8 and the Metro UI, on the other hand, is just a match made in hell. I used it for five minutes on friday night and spent all but 10 seconds of that time trying to work out how to go from IE back to the desktop and failed miserably. No command keys worked and there were no visual cues in the UI to help me out, so in the trashcan it went.
Contrast this with the way Apple are grafting iOS UI components in to Lion and the upcoming Mountain Lion and the contrast is way too extreme