For me, mostly, it's the physical design of the machines. Sure, Apple has all this shiny aluminum stuff now that looks very modern and whatnot, but nothing they offer these days comes even close to the pure beauty of the PPC era. Try and find any computer on the market today that looks like the clamshell iBook G3, the iMac G4, or the PowerBook G4. That's right, nothing. But you see tons of MBP look-alikes, as well as other Apple designs. I admit, there were a few competitors back in the day that tried to emulate the PPC design, but for the most part they failed miserably. Put a MBP and a high end HP laptop next to each other, it's hard to tell the difference.
Another thing is, Apple no longer offers any computer I would consider purchasing. They all either have something wrong in the cosmetic design, or hardware design that I just can't overlook. The MacBook was the last machine I would have thought about buying, and they took that away. So, they no longer have my business.
But, even if they had something I liked, I sure as hell can't afford a new Mac anyway. Not even close.
And finally, as others have said, there's just something about those old machines, a feeling you get from looking at and using one that just doesn't happen with the new offerings. The PPC Magic, as I call it. I still get more of a thrill from turning on and using my eMac than I do playing around on all the new machines at an Apple store. I don't know if it's really just the looks, the different processor, nostalgia or what, the new machines just feel cold and hard in comparison to the loving warmth of the past.
I own a few retired PPC's, and my eMac which I still use everyday. I hope to never have to replace it, it's built like a tank, so at the very least it should last a while yet to come; but should that day come, you can bet I'll be getting another PPC, perhaps an iMac G5 or a similar PowerMac. And I suppose when the PPC platform becomes unable to cope with the simplest tasks I require, I'm getting a PC unless Apple has done something to bring back the magic. With the iPhone and iPad being their main priorities, of which I have zero interest in, I can't see that happening. Such a shame, to see what was once unique and full of life become neglected and left on the side of the road to die.

Cost of progress I suppose...