I suspect that the Start Screen and the apps probably make better sense on a laptop, which has a screen between eleven and fifteen inches wide, than it does on my monster 27-inch screen. As an example, when you’re using an app, if you want to close it, you swipe downward on your screen from the top. Probably not a problem on a small screen, but on my Dell XPS One? That’s a whole lot of real estate to drag through, and it quickly becomes impractical. As do fullscreen only apps, which would be more practical and useful in a smaller window. This is one of the reasons why on a day to day basis I don’t use any of the apps at all and stay in the desktop environment almost exclusively.
This is true of Mac OS 10.7 and 10.8 as well: absolutely optimized for the laptop, rather than the desktop. Maybe not in quite as annoying a way, for desktop users, but the fact is that a desktop user of OS X has on net not really seen much improvement from the last couple of iterations of Mac OS.