1. Web development. You CAN NOT develop for IE on a Mac unless you either have a PC or have memorized every conceivable compatibility problem between Explorer and.. everybody else
2. Mac versions of PC software tend to be kinda chintsy, at least in my experience. Office v.X is ok but there *are* certain quirks between it and the industry-standard PC version. And then there's the Mac IE.. *shudders*
3. Considerably less choice when it comes to hardware and hardware upgrades. But thank god for USB and Firewire
4. Price, of course; and related to that, 'bang for your buck'. My dad just bought a brand new 3.2GHz PC for $700, monitor included. That was a steal, but it's not unheard of. Apple doesn't even offer speeds that high, and to get close to it you'll have to shell out $3000. Blahblahblah, megahertz myth, come on: deep down you all know it's true that Apple's behind (as, indeed, the folks who've received the Developer Macintels have now confirmed...)
5. If you're expecting a cure to all that ails you in the PC world, think again -- Macs do crash, they can be painfully slow, and their software is sometimes buggy. The lack of viruses/spyware is true (then again, it's also true that it's very easy to run a spyware-free PC without much effort. The key: completely ditch Internet Explorer and any program that uses the IE browser control), but a lot of the other stuff you hear about OS X is just hype.
6. As far as software goes, backwards compatibility is practically a forbidden concept. By way of contrast, old shareware programs written for Windows 3.1 in 1992 still run perfectly fine on my XP SP1 laptop (and yes, I still use them, dadgummit!).
7. Every year and a half when Apple releases a new minor upgrade to OS X they treat it like it's a brand spanking new OS -- and expect you to pay for it. To a longtime PC user, this sounds utterly ridiculous and inappropriate. Tiger is little more than OS X Service Pack 4, with some cute bells and whistles thrown in.
8. iLemmings, and the possibility of being perceived as one
..oops, that was a little more than 5. Well, as you can tell I'm not totally enamored of Apple. Don't get me wrong: I could probably write a list thrice as long for reasons TO switch, as I myself will be doing just as soon as they can get their act together and release the next Mini. But let us not be blind to their faults.