That makes me think about something. When was the last time you can remember that people went out and bought a new PC or Windows OS because a new version came out? Was it maybe, Windows 95? Computers are not following the trend the phones are putting forth, where people will go and buy a new phone when it comes out. This is not just an Apple thing, as I have known people also being excited about new Samsung or HTC releases, although they just don't make the news.
I would say that, most of the time, a person is buying a new computer because their current PC is too slow or has failed and needs a replacement or that they want to get a notebook for the kids, going to college or a work requirement. Even corporations are only buying computers for new employees or to replace existing computers. For the most part, the consumer has no real choice as for what OS comes on their computer. Corporations have a greater control as they may have a volume license.
So in my mind, any changes to the OS can't possibly to make people go OOO and AHH and want to buy it. The changes must be done for some other reason... which might just be because they can...
And regarding the reason why anyone really cares about which OS version they are using, it comes down to stability and friendliness. XP and 7 were pretty much rock solid. Vista had a bad start at the gate due to various reasons but eventually ended up just like 7 post SP1 but still many people think it is horrible. Windows 8 had a bad rap, solely because of the GUI change although there are quite a few other reasons why people don't like it. People who actively use Windows 8 on the desktop, and not using the tablet/portable GUI find the OS is just fine. But you will fight those first impressions for the lifespan of the product.