Apple's philosophy is to create hardware that's appropriate for a particular use, run appropriate software on that hardware, and (with the advent of iCloud) sync all your documents and data to whatever device you're using at the time. All of Apple's stuff is clearly part of the same family, and follows many of the same conventions, but a MacBook Air just isn't an iPad. Even if it ran iOS on top of OS X, it still wouldn't be an iPad. iPads run apps meant to be used on tablets. iPhones run apps meant to be run on phones. Macs run apps meant to be run on laptops (and sometimes desktops).

Microsoft is doing this its way. The entire approach here is uniquely Microsoftian, at a time when almost everyone else in the tech industry is trying to take a page from Apple's book. Perhaps it'll even work this time. But as someone who was excited to see Windows Phone 7's Metro interface come to a tablet device, I'm disappointed, because it seems what we're getting is a small Windows PC with a tablet-interface shell floating on top. It just feels like the wrong approach to me, but I'll say this for Microsoft--it's consistent.