Some theory behind Mac OS X's menubar
Windows keeps the Start button, taskbar and system tray at the bottom of the display and a menubar in every window. Mac OS X keeps one main menubar at the top of the display, with a 'dock' of larger icons that take the place of the Windows taskbar at the bottom of the display. Linux, for the most part, seems to prefer the Windows UI, typically using a taskbar-like system with menubars again in every window, but through the power of Open Source, you can do just about anything you want to the Linux UI to make it feel more like home. Some people find one approach more useful, while others prefer a different side of the fence. While the debate surrounding one's OS preference isn't showing any signs of subsiding, we thought it might be useful to offer at least a little insight and theory into why some fundamentals of Mac OS X are designed so differently.
One of the basic principles that informs the Mac OS X menubar is something called Fitts' Law, which I first learned about from John Gruber of Daring Fireball in a post here. To keep things brief, however, I'll just quote a short introduction from the Wikipedia:
In ergonomics, Fitts' law is a model of human movement, predicting the time required to rapidly move from a starting position to a final target area, as a function of the distance to the target and the size of the target. Fitts' law is used to model the act of pointing, both in the real world, for example, with a hand or finger and on computers, for example, with a mouse.
To summarize: Fitts' Law is about how far you have to travel to hit a target, and how easy that target is to hit. Apple implemented these concepts (and I'm sure plenty of others) when designing their menubar by pinning it to the very top of the display, not only from a hierarchal standpoint (you can always look to the very top left of your display to find out exactly which app you're in), but also from a 'make it easy as possible to hit this' perspective. You can simply fling your mouse 'up' and you're at the menubar; even if you click on the very top-most pixel above File, Edit or Help, you'll still hit that menu item and activate it. It's a seemingly minor detail, but one that can help quite a bit during one's daily computing.
This concept is also present in other major OS interfaces, such as the Windows Start button; fling your mouse 'down and left', click and you'll hit the One Button to Rule Them All. Mac OS X's Apple and Spotlight menus also function the same way: fling your mouse 'up and to the left', click in the furthest pixel up there and you'll activate the Apple menu; 'up and to the right', and you're in Spotlight.
If anything, the main point we want to get across is that there is typically a lot of theory that goes into the design of an OS and how users interact with it. We might not always agree with the approach taken by one camp or another, but at least people are thinking about this stuff, because even in 2007, computers still aren't that intuitive to some users who have yet to hop on board the digital train. The more thought, consistency and intuitiveness OS engineers design into our software, the easier it will be for everyone to come along for the ride, no matter what side of the car they're sitting on.