For me that feature is Spaces.
My use of Spaces has become highly tuned now. On my dual screen Mac Pro I have 6 Spaces and keep certain types of application targeted in each of them. Here's how I use them:
|1) NetNewsWire||2) iTunes|
|3) Safari / Adium||4) Open work area|
|5) Rails development area||6) VMware Fusion / Windows XP|
Which means that when activated it looks like this:
I have mouse button 6 on my Logitech mouse dedicated to Spaces so I can quickly navigate when I'm in "mouse mode", I have F5 dedicated to Spaces on the keyboard when I'm in touch type mode and I use either Control-Option-Arrow to move between spaces quickly or hit Control-Number to pop directly to a window. If I'm alternating between two applications quickly I'm hitting Command-Tab.
On my MacBook with its limited screen real estate Spaces is even more important.
What's your favorite?
So there you have it - my most used and valued feature of OS X - the one I would have a really hard time without. Apple says there are over 300 new features in OS X - if you had to choose one single feature on your Mac (Finder doesn't count!) that you couldn't live without, what would it be?