If you compare the two operating system's folder icons, you can already see a few key differences. While both of them clearly sport the traditional folder shape, the Windows 7 icon uses the traditional yellow colour, a colour which is reminiscent of actual, real-world folders. The current folder icon for Windows clearly evolved from the folders used in Windows 95. Mac OS X opts for a speckled blue, reminiscent of recycled paper; a change in Leopard which not exactly met with universal praise.
Windows and Mac OS X also use different methods for indicating folder type; the later embosses an image on the icon, while the former opts for an emblem sticking out of the folder. I personally dislike the Mac method quite deeply, as it can be quite difficult to tell one embossed blob on a folder apart from another. In the pre-Leopard days, the labels were coloured making them much easier to distinguish.
Mac OS X has been using more or less the same type of drive icon for a while now, while Microsoft introduced new icons in Vista which carry on in Windows 7. The Mac OS X icon is clearly an internal drive, while the Windows one looks more like an external one to me. For those who are interested: yes, Windows still contains drive icons for not only 3.5" floppy drives, but also 5.25" floppy drives.
When looking at the icons used for empty and full recycle bins, the difference in visual style becomes quite clear: Windows is clearly all about the whole glass thing, while Mac OS X opts for a more natural-looking trash bin. When full, the bin in the Mac icon also bulges a little bit (see the red arrow) which further illustrates the fact that the bin is, indeed, full. If I'm not mistaken (please correct me if I am), the trash icons on the Lisa and the original Macintosh did the same thing. The Mac icon is also a bit odd as the 'webbing' on the backside does not continue below the front rim (you barely notice at normal size).
I chose to compare the editor icons in Windows and Mac OS X to illustrate two specific points. First, while Windows 7 clearly continues the 'glass' style introduced with Windows Vista, it also comes with several icons in a new style, which makes quite a few steps away from the glass idea. The top left icon is Notepad in Windows 7, and it's clearly in 'glass' style, with the transparent front cover. The bottom left icon, labelled 'new style', is the WordPad icon in windows 7, an as you can see, it follows a different style. The control panel and Paint icons sport the same new visual style (and there are a few others scattered throughout the operating system), and if my memory serves me right, this new style did not arrive until the RC build.
The other point I want to make with the text editor icons is a bit more philosophical. As you can see, TextEdit's icon contains text - this is the text used in the "Think Different" TV commercials Apple ran in the late '90s. Little personal and playful touches like this can be found on many icons in Mac OS X, and they illustrate a key difference between the Windows and Mac icons: Windows' icons are mostly strictly utilitarian, while the icons for the Mac often have a more artistic touch.
The fun part of course is that this relates very well to the bigger picture of the two operating systems: Windows is still mostly business-oriented, whereas the Mac is more personal, more artistically oriented. Of course, this is not a law; it's merely a general observation which you can see shining through even in the icon sets.
It's fun to see that both Windows and Mac OS X take a clear utilitarian approach to icons used in the control panel and System Preferences. The icons used there must convey their meaning clearly and directly, with little to no room for misinterpretation. Apple obviously draws from its own hardware product portfolio whenever it can.
This was of course just a small selection of icons, but it's interesting nonetheless to see how the two opposing sides look at icons. I do hope that Microsoft switches away from the horrid .ico format, and just stores icon files in glorious 512x512 .png or something, as handling .ico files stuck within .dll or .exe files is a total nightmare.