Reasons to not go with a Mac? That's the official topic. Hmmm...
We'll I've certainly used both for many, many years. The biggest downside to the Mac laptops that I can see is the lack of an integrated, two-button mouse. It's a non-issue for the desktop.
On the desktop, I personally think dual-booting just isn't much fun. If you're not worried about gaming, this is a non-issue as well because Parallels desktop allows you to seamlessly used Windows apps from the Mac desktop. Dual booting is a hassle, however small. If you are a hard-core Windows gamer, a Mac just isn't going to be the right call for you, for this, and many other reasons...among which:
Upgrades are another big reason, as many have mentioned. The iMac's biggest strength is also it's biggest weakness: it's design. You're stuck with dead pixels or a failing screen after the warranty ends. The quality of LCD panels has increased sharply so this isn't nearly as much of a con as it could be, but it sure would stink to have to pay Apple through the nose in order to get a 20" LCD screen replaced, when comparable models cost $200 from Newegg. You also won't be able to easily upgrade the GPU, even if there is space...there aren't proper BIOS and drivers available for most cards on sale.
Also, if you don't get enough memory the UI can be sluggish. It's probably just that my experience with OSX has mostly been on my wife's powerbook G4s, but the UI can be somewhat less responsive on lower-end Mac hardware, especially when compared to stripped down version of XP.
Finally, someone already mentioned the dock as being inferior to the Windows taskbar, and on some level I agree. The dock can be very limiting for some types of power users. Essentially, if you use a very small number of applications incessantly, the dock is for you. If you have tons of applications you use consistently, but often infrequently, then the dock is a poor solution. In the latter case, it becomes crowded and less easy to see, especially once a lot of applications and windows are open (as they are added by default to the dock). To get an idea of whether or not you'd like the Apple dock, I'd recommend anyone interested download ObjectDock from Stardock.com. It gives a good (if much slower and less well-integrated) look at the Mac's dock functionality.