Hardware-wise, nothing. They're both made from the same cheap chinese/taiwanese components. Except Apple charges morefor those components than everyone else. The only real difference is in software, but that's not a PC vs Mac issue, it's a MacOS vs Windows issue. No one is forcing you to use Windows on your PC, after all.

If you want to compare MacOS version whatever and Windows NT, the following applies:

First, Windows has got, by far, the largest software library. Including a lot of applications many users think of as impossible to live without :p. Of course, you can say that it's possible to emulate Windows programs in MacOS, but that's a pretty pointless discussion. Then you can argue for there being tons of games on the PSP, because it can emulate SNES, NES, GBA, et cetera. Also, emulation is usually less efficient and not always as stable as running the application on its intended system.

Second, Windows's "lol crash" reputation is a bit unfair, I think. Considering the majority of BSOD problems were found in Windows versions that weren't NT based, such as Win95, 98 and ME. I'd argue that the vast majority of BSODs in windows NT 5.x/6.x are caused by hardware problems, and if it is caused by software, it is almost always a kernel driver problem. And Microsoft aren't the ones writing drivers for all the hardware out there. Now, this leads to another interesting point, which is sort of double-edged.

You see, Macs, while using basically the same hardware as PCs, are more closed off. Apple doesn't need to support as many hardware devices as Windows does, because Macs are built with a few but (probably better) tested components, and their users rarely, if ever, replace any of the hardware inside it. Because of the lower amount of hardware devices, a lot fewer drivers need to be made too, and that frees up time to test each driver much more thoroughly. And all this leads to more stability.

A PC with Windows however, especially if you're an above-average computer user, needs to support an incredibly much larger number of different components, and to ensure that they all work nicely together. Some of these devices don't even have drivers that have been checked by MS to be "good enough", and some of these might not like some other component in the PC at all. This leads to instabilities, and for the average user, it looks like it is the OS which is the problem.

MacOS could easily run on any ordinary PC nowadays, because they use the same processor architecture. The only thing that stops you from doing this is that Apple puts a small chip on their Mac motherboards, where it is written "This is a Mac" or something. If MacOS doesn't find a chip with this text in it, it refuses to work.

However, people are working to find a workaround for this problem. I expect MacOS running on a PC to "magically" start encountering more strange problems than when run by a computer built by Apple, because of what I said earlier about MacOS not normally having to troible itself with a lot of different components.

A nice analogy could be:
Windows-PC = Driving your own car.
MacOS on Macintosh = Taking a cab, or hiring a private driver.
Linux on anything = Building your car from scratch, and driving it yourself
MacOS on PC = Hiring a car driver to fly your F-22 Raptor figher jet.