One of the first things I noticed when I switched to a Mac was the font rendering.
It's going to seem like a really stupid thing, but it was the difference in quality of the characters. A relatively small detail you might think, the particular antialiasing algorithm that determines what shades of grey make up the pixels around the edges of each character. The result is a smoother, darker and less jagged appearance to text on the screen that may seem relatively insignificant until you consider that it affects every screen, task and interaction (or close to it) to you make.
It's actually pretty important in a medium where so much of our content consumption is text based. For all the multimedia awesomeness of the modern internet most of it is still text. The news, blogs and so on all rely primarily on text with other media for support. Very few sites manage to promote video or audio as their primary feature - they're simply not a suitable medium... they dictate the speed and environment of consumption where as text is entirely at the readers leisure. Convenience, as you might expect, is the decisive factor.
This captures perfectly for me the difference in philosophy between OS X and other operating systems. Only recently has Microsoft started to pay attention to the quality and aesthetic of it's interface. Before Vista/Windows 7 it was hard to see that there was a great deal of consideration for ergonomics either. These considerations are fundamental to the design of Apple products however. They're not necessary for the successful operation of the device, but if all we chose in life was based on utility the world would look somewhat different.
So every last pixel really does count and can make a difference, and every small difference matters.