* sandboxing applications/programs - so they can't just access the internet or make changes to important parts of the operating system or your own data.
* randomises the location of system libraries in memory while you are using it. This makes it incredibly hard for hackers to take over the system, especially if they can't find it.
* central station - more efficient use of multi cores and processors. A single application can have every thread and process divvied up between all available processors. This is not just threads but the most minute task or computation. Think of it like the way the internet breaks emails up into minute parcels to break up the workload. OS X does this with everything.
* sends as much data as possible to the graphics processor unit (GPU) to take the load off the main processor (CPU). The GPU is often more powerful than the CPU yet sits dormant most of the time. Apple takes full advantage of this extra power.
* GPU handles all graphics tasks, even simple 2D drawing such as the desktop and windows.
* The file system defragments on the fly as you use it. You will never have to run a defragmentation utility ever.
* save a version and revert to a saved version - you never have to save anything. It automatically saves every version of every document you create. You can go back to any change you made to any document and change it, keep it or start borrow from it for your latest version or a completely new version of your document.
* time machine - automatically backs your entire system in the background. It doesn't just restore your system. You can go back and pick and file or thing from and time and bring it back.
* OS X will determine on the fly what it needs to do to make most USB devices work without the user installing any drivers. It has entire classes of drivers built in, and it can pull the information it needs in order to work directly from the USB device itself. This is why you can plug in cameras, any mouse, joysticks and other devices and start using them instantly even if they don't explicitly say they can work with a Mac or OS X.
* multi touch gestures - apple bought the company that invented them, and used them throughout the operating system. Gestures can be everything from the innovative pinch to zoom to using all five fingers with a trackpad to do everything a mouse can and way more.
* spotlight search - introduced way back when is still the most advanced system search around. It can search down to the meta data and hidden information in any given file not readily available to the user.
* PDF document format has been built into the core of the system from the word go. You don't need adobe acrobat or any kind of editor because OS X "prints" natively to PDF format. It uses the protocol everywhere. It is an integral part of the system.
* quickview - allows you to preview a document on the fly at anytime, anywhere, even if you don't have the program installed that created it. Just hit the space bar on its name and and you can view it. If it's music you can play it. After you have used it, you will wonder how you ever got on without it.
* App store - the advanced thing about this is that once you buy something, you never have to look around for activation keys, or installation disks for reinstalls, or worry if you can install it on your second computer. If you buy a new system, you can just reinstall all your software from just one point. It is simple and yet makes moving between computers very easy.
* parental controls - these aren't just for kids. It is a great way to make user accounts and control to the Nth degree just how much power you want them to have. If you do have kids you can dictate not just what they can use, but restrict the hours the computer can be used, and end even have it email you to confirm who they can talk to online through chat and their own emails. There is a lot lot more than can be described in this space.
* dialog boxes are tied to individual windows. So when an event happens your entire system does not lock up waiting for you to do something. If you are working in a different window you can just keep on working until you ready to deal with it. You can also just keep on ignoring it.
* universal access doesn't just help the disabled. It can also read out dialog boxes hidden behind other windows and events as they happen even if you can't directly see them. This means you can ignore them too.
* it is certified UNIX under the hood. This has been the most advanced networking operating system since it was invented in the late 1960s and continually polished and upgraded to where it is today. UNIX is what the internet is built on.