Theres more to supporting Aero than just writing the code. they have to actually test a majority of the hardware that people actually use. The more of it there is, the more work needs to be done. And theres a heck of a lot more out there in PC land.

Even in the minimal case, the PC has to support 6 times more variants than the Mac does.

Technically, Linux supports more than that but I don't remember it having too much trouble
There have been some problems with drivers but they are mostly resolved after drivers get fixed.

If AMD/ATI would fix their drivers it would be much better. Or is it only a coincidence that every single time I've installed NV video cards in my PC's they have worked? Also, OS has nothing to do with it, it is purely driver creators job to make it work.

It is, just that 90% of market share is good enough reason to make drivers that work

I'm very late to the conversation, but I know quite a bit about OS X and Quartz 2d, and have used it on a decent spectrum of computers, so...

OS X v10.0 and v10.1 don't really use the GPU much at all. They're technically more advanced than XP because they buffer much more (so, for example, when you move windows around you never see the ones underneath hastily redrawing themselves, or if they're not responding because they're busy or about to crash then simply refusing to draw in any graphics), but GPU-wise they're not really doing much. Transparency and such like are heavily used and hardware accelerated OpenGL is available but that's about it. Of course there are other fundamental differences, like the way that OS X's drawing layer is based on a version of the PDF standard whereas the old XP way of doing things is GDI+ — which is quite proprietary and comparatively limited.

As of OS X v10.2, more modern GPUs (= anything with at least 16 mb of VRAM and texture rectangle support, which I think means GeForce and Radeon onwards) can be used to do hardware window composition. So the CPU updates window contents using PDF stuff then the graphics card puts all the windows together on screen, does all the transparency and handles all the moving around and stuff. That's why traditionally in OS X window moving has been a lot smoother than window resizing, but you could partly argue that's a clever trade-off in an OS that doesn't do child windows and most apps work by providing one window for each document with just the document in it, one window with controls and, of course, the menubar always in the same place.

In OS X v10.3 they introduced Expose which is arguably the most blatant use of hardware compositing in the current OS. It uses the hardware to do clever things like shunting all the windows around when you're searching for one — e.g. scaling all the windows down and arranging them on screen so you can see all at once. Then you click the one you want and they all return to their normal sizes but that one is topmost.

As of OS X v10.4 there is a disabled feature called Quartz 2d Extreme, which does most of the window drawing stuff on the GPU provided it is glslang capable (which I think means PS 2.0 in DirectX terms). You can turn it on using the developer tools, but it isn't on by default. Apple haven't said anything, but the assumption is that it is still buggy and will probably be a proper feature in OS X v10.5 which is due this spring.

They've been using the GPU to do lots of eye candy in the form of transitions and other things for quite a few years. See, for example:

Exposé (OS X v10.3) demo
Dashboard (OS X v10.4) demo
Time Machine (OS X v10.5) demo

I've used OS X v10.4 on a 450 Mhz PPC with no acceleration, a 667 Mhz PPC with Quartz 2d acceleration and a 2.16 Ghz Core Duo with support for Quartz 2d extreme acceleration. Window resizing is snappy on the final machine, exposé and general window composition is snappy on both of the last two machines, window movement and general functionality is snappy on the first machine but resizing is quite a chore and exposé doesn't really animate.

Apple uses the Extensible Firmware Interface, an Intel invention that is like the BIOS but not steeped in 1970s technology (Boot Camp provides a soft BIOS for booting Windows and other, old fashioned OSs). All hardware needs to have an EFI driver but otherwise is identical.

Historically they've used most GeForce and Radeon chipsets. Now they've moved to Intel they support the more recent Intel integrated chipsets too. The only machines with integrated chipsets are the Mac Mini and the education market iMac. A new implementation of OpenGL released in an update to OS X v10.4 can effectively run the OpenGL driver on one CPU core while a game runs on another which helps a lot with the integrated chipset but at the minute software has to specifically request that OpenGL driver. Don't ask me why, maybe it's just not properly certified yet.

Early OS X supports any PowerPC. In OS X v10.4, you need a G3 (which is c. 1997/8 technology) or later but at least a G4 is highly preferable because the vector unit is very useful for the software compositor. It is speculated that v10.5 might drop G3 support and work only on G4s upwards. The last G4 was phased out last year with the Intel switch so G4 support is likely to remain for a while.

Per Ars Technica, OS X has increased in speed with every new release but officially stated memory requirements have also gone up quite a bit too.

My gut feeling on Aero versus Quartz is that Quartz is still "more advanced" because it's bottom layer is PDF based and PDF is a proper typographic industry accepted standard which has been rigourously thought out as a means for accurately presenting and reproducing information. However in terms of eye candy Apple have traditionally limited themselves to transparency and the odd 3d effect because although their engine supports more they have low end computer to think about. Those of us with glslang capabilities get some nicer effects but no more functionality. I can't imagine that'll change too soon though Quartz 2d Extreme, when it is officially enabled, should do what you expect: make the GPU do more if it can otherwise let you do as much as possible with the CPU.

EDIT:

OS X is based on Nextstep, which is itself a 1980s branch from BSD. Besides Intel & PowerPC it has also in the past been available on PA-RISC and SPARC. I also gather, if I believe wikipedia that it "was among the first general-purpose user interfaces to handle publishing color standards, transparency, sophisticated sound and music processing (through a Motorola 56000 DSP), advanced graphics primitives, internationalization, and modern typography in a consistent manner across all applications."

But Apple now essentially use commodity hardware for the internals, so they pretty much end up with normal PCs anyway. Early Intel Macs had a TPM chip that was used for OS startup only, it seems modern ones don't.

Hmm! Interesting, all I know is my humble G4 iBook has a Mobility Radeon 9200, I think its 32Mb and it seems to handle the MacOS X eyecandy with no trouble at all!

As I understand it the Mobilty cards were not the same as the full Radeons either
though I'm not sure exactly what was stripped out.

They should be the same as the "real" versions, only with lower speeds and as in this case, fewer memory. They should have all the features as the non-integrated ones.

You should get all the twirls and zooms and so on (even my 16mb Mobility Radeon equipped 667Mhz Powerbook does them), but presumably not the various Core Image and other pixel shader effects, such as the ripple when you drop new widgets onto the dashboard?

My PPC Macmini lags like crazy. It's not nearly as nice as a similar priced Windows machine.

Yes, according to its properties:

• CPU Speed: 1.42GHz

• L2 Cache: 512KB

• Memory: 512MB

• Bus Speed: 167Mhz