It's an evolutionary change, but it's a substantial one. Look beyond what Win 8 does with the desktop, because so few apps take advantage of the interface. It's changing to a one app at a time on the screen, which I'm not sure is great, though you can somehow get two split on the screen on the Surface tablets. I haven't tried that on the desktop.
However Win 8 is more revolutionary. If you look at Android, it basically copied iOS and then improved and added things. In many ways, I think Android 4.1 is far beyond iOS6 and introduces ergonomic changes that are not only smooth, but very helpful. However WP8 is a dramatic change in how things work. It's a completely new way of looking at an interface.
It's not that tablets replace desktops, but that they are a new way of doing some work. There are lots of people that can do most, if not all, of their work on a tablet. There are tons of people using tablets with keyboards, which is different than using a laptop. I'm not one of them; I prefer my laptop or my phone instead.
However the introduction of tablets and the ability to move from a tablet to a desktop and back, doing the same work, is important. Tablets work better than phones for lots of situations. I don't know that I would use one, but I also don't think it's a simple yes or no for the way the format, and the OS, works. Personally I think the OS is becoming less of a big deal, and therefore I don't understand the MS insistence of making the same OS work on both. I'd like to see OS's optimized for each format, and my apps and data slowing smoothly across either.
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