Usually when using a Desktop, I look at the screen from going top --> bottom. On my tablet, I see it more as bottom --> top. About 1/4 of the way down my monitor is where my eye level is, so I naturally look more at the top of the screen. On the tablet, stuff at the top is undesirable because when using the top of the screen, your hand is blocking most of the screen. Buttons on the bottom allow you to see the screen, and see notifications that the buttons give, while pressing them.

Having said that, all OSes I can think of have some kind of bar at the bottom. Windows has the taskbar at the bottom, OS X has the dock at the bottom, and both KDE and GNOME allow you to switch programs using a bar at the bottom. I realize these are all configurable options, I'm just looking at the default setup here.