Tough question. There is so much that goes into an operating system, and if we allow the term to encompass the range of desktop Unix-like systems to Windows and Mac OS X (not server), then you have a start.
I'm not someone that is sold on any technology we currently have sticking around until the day I die. Mice will be replaced by touch screens...keyboards might be as well....they'll both be replaced by sensors reading our eyes, and eventually brainwaves. That said, I'd limit myself to generalized features of ideas and concepts, versus actual implementation of them.
User Interface. In my opinion, that is the largest single factor in consumer (not commercial) software and operating systems. We talk a lot about fewer clicks, faster rendering, transitions, dragging, etc. They're all specific instances of a well-planned user interface. This is what has limited the adoption of Linux for the majority of consumers, and propelled Apple within the last couple of years.
Functionality. This is probably overly generalized. Obviously, we need some sort of functionality for this UI to be of any use to us. Typically, commercial applications are more focused on functionality than UI, likely because of the ignorance of the money-handlers (upper management). This is where Apple, Windows, and Linux really start differencing themselves the most...
Apple has made it their mark to bundle their own applications into the OS. They do so in a way that everything is very integrated. iMail, iChat, iDVD, iMovie, iSync, Address Book, iTunes, Safari, etc. This gives Mac's a lot of extra functionality that other systems don't usually have.
Microsoft has noticed, and Vista does include a lot more similar applications. Windows usually doesn't include nearly as many Microsoft apps, instead allowing the system manufacturer to provide them (probably not a smart decision since it divides the consumer to become manufacturer loyal, rather than MS loyal). This means that the functionality of Windows varies greatly from system to system, though they all include basic applications (Calculator, Notepad, Wordpad, Movie Maker, IE, etc).
Linux is at the opposite end of the spectrum. There are likely more applications for Linux than any other system (though not necessarily commercial or GUI), but VERY few come bundled, comparatively.
So you have a very wide range of different ways of creating functionality in your system. While I prefer Windows for most of my activities, I actually think the integration method that Apple has fostered is the way of the future, which is probably for the best. While it may reduce competition (though Safari doesn't seem to have done anything to FF) it will help secure computers by encouraging users not to download unfamiliar programs from web since they already have them.
As far as other features of an OS, it needs to be compatible with other systems, respond quickly, support a wide variety of hardware, be able to scale intelligently, be secure, effectively utilize hardware, and provide constantly improved (yet backwards compatible) methods for common operations (IO and memory primarily).
I think the single largest feature that makes a good operating system is creative and innovative developers and designers working together in a relaxed atmosphere where they have considerable say in the end product. Capitalism fails when consumers aren't educated about the product. 😉