They're just a more controlled environment really. Windows computers do just get clogged up with all sorts of random crap if you don't pay attention...Apple take a lot more control over the Mac 'experience' so you do get something that works a bit more smoothly. The penalty you pay is that they are a lot less customisable, a lot more expensive, and some things just aren't available for them at all.
And of course a lot of windows PCs are sold with hardware too slow to ever run them well, while Apple have a minimum specification for their cheapest (relative term) computers which does give a reasonable minimum of performance with their software.
I personally still feel it's a ripoff - my boss has just spent £2.5k on a mac pro which is way outperformed by the PC I've built myself (total cost of components over the years <£1000), and while OSX has a few neat tricks (like a backup utility that just works, without any thought,, a sleep mode that *works* (he hasn't actually restarted it in two months - try that with a windows PC!), and a tendency to never actually *crash*, just to kill individual programs when necessary), it's also bloody annoying to use in a great many ways.