I'd like to preface this post by saying that I am not an OS level programmer, though I understand the fundamental abstraction layers in most modern OSes. On the other hand, I have a strong background in UI design, and consider myself a proponent of Clayton Christensen's "jobs to be done" theory.
Consumers hire OSes for 4 reasons:
1. Manage applications
2. Manage data
3. Aesthetic and functional preferences
4. Manage connectivity
Let's break down each one in detail.
1. Manage applications. This is without a doubt the single most important reason why consumers hire OSes. Microsoft didn't rise to prominence because it built the best OS. Microsoft wooed developers and attracted businesses who had problems that needed to be solved. The iPhone didn't really take off until it had the App Store because App Store enabled the iPhone to do so much more than Apple could ever on its own. Apps are and will be the key to all computing platforms.
It's interesting to note that iOs is also the worst modern OS with respect to allowing apps to share information with one another. The concept is nearly nonexistent in iOs. Given that application management is the most important function and biggest driver or purchasing decisions of modern OSes, this is a competitive threat for Apple. Consumers will eventually demand application and data interoperability that iOs doesn't provide today.
2. Manage data. This key tenet is the most interesting with regards to diversity of implementation in modern OSes because iOs, the pioneer of modern mobile OSes, completely abstracts the file system from the user. This stands in stark contrast to all other modern OSes. Apple's approach to data management is to integrate the data so seamlessly into the applications that you don't even realize that you're managing your data. This strategy has worked wonders in the mobile world where user inputs and screen real-estate are extremely limited. In the desktop world, consumers need more advanced data management tools like file browsers.
In the desktop world, the OSes have nearly perfected data management. Windows and OSX handle data management with ease, and allow applications to share data easily. Android is also competitive in this front, though the UI is much more limited, making its usefulness significantly less pronounced. Chrome OS takes perhaps the most interesting approach to the data management problem by abstracting it away to the cloud, accessible via apps, with the notable exception of Google Drive.
I would also throw data back-ups and security into the data management "job to be done", as they are a crucial aspect of data management. iOs probably does the best job here of all OSes because it's locked down, making it quite secure, and because OS-level data backups are available for free to all iOs consumers through iCloud backups. No other OS vendor has ever provided this level of data backup solution out of the box, seamlessly integrated, for free.
Overall, data management is probably the area with the most differentiation among modern OSes. Data management is also probably the least talked about, interestingly enough.
3. Aesthetic and functional preferences. There are 2 broad classes of modern OS UIs - desktop and mobile. Within each class, all the major OSes have achieved UI feature parity - they utilize these same fundamental UI paradigms (windowed apps with a keyboard and mouse, and capitative touch, respectively). But of course there are subtle differences in aesthetics and functionality among the major OSes, and consumers may develop preferences for a certain OS, despite the fact that other OSes can do the same thing. Some consumers prefer iOs's decidedly simple (and limited) grid of icons, while others prefer Android's more robust home screen widgets. On devices with limited screen real estate, I actually prefer iOs's limitations, even as a power user.
In some cases, consumers hire an OS because of aesthetic preferences. Some consumers just really love bright red plastic cases on the Lumia 920. I personally drool over the build quality and design of the Retina MacBook Pro - it's absolutely superb in every way that I need it to be.
4. Last, and most certainly least, OSes manage connectivity to 3rd party hardware, and most importantly, the Internet. All modern mainstream OSes are well beyond "good enough" in this regard. Still, for some consumers, hardware connectivity for specific devices or ports may be OS-limited, thus driving OS purchasing decisions. These cases are few and far between, and becoming less common as an increasing number of devices use common connectivity standards such as USB, HDMI, Bluetooh, Ethernet, Wi-Fi, and Thunderbolt.