@DeathByVisualStudio: different demographics, I suppose. My point was that even vastly successful Windows versions had to face some sort of "upgrade friction": you always need a compelling story to get people to upgrade, and an even better one to get them to upgrade early.
Not sure what WP7 has to do with all this: one thing is selling an upgrade, anotther thing is to get people to switch platforms. If anything, WP is unsuccessful despite the fact that it offers a great UI: it just doesn't enable you to do anything you cannot do with the other devices. And it's just not cool, but that's a different story.