the GUI insides...
Just to add my 2 cents...
I have been using XP since the preview period last summer, and OS X as my primary OS since September. I feel that I have a pretty good handle on what each one offers.
XP is referred to as the "Fisher-Price" OS by many techs in the business. The first thing that most of us do is to turn on "Windows Classic" theme so that we don't have to see it. Plus you gain better performance by turning off the "features" of the Luna theme. The GUI is a resource hog!
The taskbar has not evolved much from prior versions of Windows, other than the annoying pop-up windows that don't go away until you click on them.
My PC system is more than capable ( Athlon 1.33 Ghz, 768 MB DDR). That said, I still have had several problems with programs crashing repeatedly in use. Dreamweaver 4, Media Player and IE ( especially when viewing QT, hmm, suspicious

).
I've also had problems where XP goes out to lunch trying to implement its "feature" of slideshow previews in folders with many high resolution images. But the OS overall has been much more stable than any of the 9x/Me series. It's about on par with 2000.
OS X is simply a thing of beauty. It's eye-catching and pleasant to look at. It's
VERY stable. I have seen maybe 3 kernel panics EVER, and those were on unsupported machines, or machines with hardware problems.
Dock functionality is way beyond the combined taskbar/quicklaunch area in XP. Overall the experince is fantastic.
However, the main thing hindering it (other than waiting for all the applications to port to Carbon/Cocoa) is the speed. If you compare IE side by side on an XP machine and an OS X machine, the PC version is much faster. Now we can argue forever about IE being built into XP and taking advantage of that fact, but the plain truth is that it is just faster. Until Opera comes out with an alpha version, we're left without many alternatives. And since for most people, surfing and email are the most common activities, this is a big deal.
In order for OS X to really take off, we need two things:
1) Faster performance without insane hardware requirements, and
2) A completly native application base. Photoshop please hurry!!!
Right now Classic feels like a tacked on solution. It's the conjoined twin (SouthPark fans rejoice) that's going to keep it's sibling from getting too many dates.
Hope my post interests some of you.
And Bobd, thanks for some good points.
Ton anglais et bien meilleur que mon français!