This is not an exhaustive list by any means ..... may do one up as a separate post some time but for now here's a quick start:

The most important thing it does is get out of my way and not constantly annoy me by asking me am I sure about this and am I sure about that and am I sure about the other. Rather than always wanting comfirmation OS X just lets me do shit but gives me an un-do option should I have made a boo boo. So, the immediate difference is less clicking.

Another huge factor for me is Expose. The ability to cycle windows within just the current app rather than all windows or cylce all apps or see all windows in this app or all windows in all apps or all windoes in this app etc. It's basially alt-tab on steroids.

Then you have all pervasive drag and drop support. On the mac you can drag and drop just about anything from just about any app to just about any other app if it makes sense. If you have a colour select dialogue open in one program you can drag and drop that colour not just to elements within that app but to elements in any of your apps where it makes sense to drop a colour.

Then you have the doc, I find it much quicker to work with than the start menu and much clearer even when you have bucket loads of apps open.

And finally for now, spotlight. Search the content of your files effectively instantly. Find doesn't have a patch on spotlight.