People often walk up to me and ask, "Dean, you indomitable man, why do you have your taskbar at the top of every machine you use?" Be they online looking at screenshots of my desktop or standing behind me awkwardly watching me working, people are often dumbfounded by my preference for keeping the taskbar (or dock, or whatever the OS in question calls is) at the top of the screen instead of the (usual) default bottom.
This is another one of those cases where I think too much about software, or at least more than casual users do. As someone who spends nearly all day working on (or playing with) a computer, I've had a lot of time to try out different methods of interacting with them to make my life easier.
Think about your most-used applications. If you're like most people the applications you use the most will be your web browser at home and your e-mail client at work (or integrated development environment if you're a programmer). Unless you're qedi, your web browser, e-mail client and IDE are all going to be graphical (meaning you use a mouse to interact with them). Most modern applications are.
Most graphical applications on most operating systems have a very similar layout: they have a title bar at the very top which is used for moving the window and closing it, a menu strip beneath that with lots of options, a toolbar (or set of toolbars) beneath that with common options, and a main working area which you're concerned with 99% of the time. Depending on the nature of the application, the right side will often have a scroll bar, the left side might have some additional toolbars or navigational aids, and the bottom will have a status bar or maybe another small toolbar.
In most applications, the buttons and menus and options are at the top of the window. When you're using the application, your mouse is usually going to be somewhere in the central, main area, or at the top of the window. If you're most users, you use your applications maximized, which means the top of the application is actually the very top of the screen. But even if you don't, the nature of the options being at the top of the application mean the mouse is probably going to spend most of its time in the upper half of the screen.
Now think about what you do outside of these applications. When you need to launch a new application, or switch between applications, or change some settings somewhere on your computer. You're going to find the application you want to use in the start menu, or you'll minimize the current application from the title bar and find another open application already in the taskbar. In either case, you'll be moving your mouse to the taskbar to access something outside of the current application.
Since your mouse spends most of its time at the top of the screen, but your taskbar is at the bottom, you have a long way to go to access its options. Keeping the taskbar at the top of the screen reduces the overall movements of this nature that you do, which can add up and really slow you down over the course of a day.
Keeping the taskbar at the top of the screen takes advantage of what UI designers have been doing for years, which is keeping options at the top of the screen. All of the functions are at the top already. Moving some of them to the bottom without moving all of them makes more work for yourself is would be inconsistent with every other application out there.
This also applies to interaction devices other than the mouse. If you're using a touch screen, it's easier to use an application the less you have to move your hand around on the screen. If you can access different features simply by moving fingers around rather than the whole hand, you'll get things done faster.
You can move the taskbar to the side, too. At least in Windows, the start menu and items in the taskbar are biased toward the top of the screen, so you get the same advantage of not having to move the mouse as much. The difference between left, top and right isn't so clear cut as the difference between top and bottom in terms of mouse movement. For example, if you have multiple monitors, it makes more sense to keep the taskbar between the two monitors rather than at the top of one, since that would take more time to navigate. If you have a narrow monitor (like an LCD viewed in portrait mode), a taskbar on the side can take up valuable horizontal space.
In the end, you should pick whichever layout makes the most sense for your situation. The bottom line is most applications favour the top of the screen, and so should you.