I'd be hard pressed to choose between Window Management and Speed and Lack of Viruses / Spyware / Adware.
Window Management
I know I'm cheating by grouping together lots of features into one but they all complement each other so well and make day to day tasks quick.
Something as simple as Command-` is one of the first things you miss when going back to Windows. Just being able to loop through windows in the current application is extremely useful compared to EVERY single window open as is the case in Windows.
Then there's the Zoom button, which especially in a widescreen display, makes more efficient use of space and allows you to have windows side by side and encourages multitasking.
Add the fact that you can Hide applications, move background windows without focusing on them and (in Leopard) scroll non active windows and productivity is greatly increased and I haven't even talked about Spaces yet ... !
Speed and Lack of Viruses / Spyware / Adware
It's just so refreshing not having to wait for the latest anti-virus update to download and install at least once a day when starting up the computer and knowing that if you let other people use your computer they're not gonna inadvertently install adware and spyware on my system.
Best of all though is that startup and shutdown times remain the same over time whereas in Windows things just get slower and slower over the months and a complete reinstall seems necessary at least once a year. In Mac OS it feels like I've got a new computer every day with everything running nice and quickly.
I was wondering if a jab at another operating system would show in the top ten seeing as how the OP asked about OSX
The trouble is is that it's hard to show how good the above two features without showing them in some kind of context and that context for a lot of people is Windows. I personally think that there are many features of Windows that are noteworthy and Apple really should have implemented as well (why do I have to go to the lower right corner of a window to resize it? That's madness when having to using a trackpad!).
But in terms of the above two it can't compete. I don't, like some people on these forums, HATE windows and find it impossible to use. I like it. I just prefer Mac OS more. Simple as that.
