What I like is that it's very dynamic. I can pull down (or pop up) a menu and while it's displayed hit the Option key and the menu immediately changes. I don't have to be holding down Option initially to see the variations.
One of the reasons I really like the OS X UI is that it does a wonderful job of providing features in a progressive disclosure model that works for me. While I was just starting out on my Mac nearly everything I needed was right there. Once I needed to do something off the beaten path I just had to dig a little bit and the next layer of options became available to me.
I realize that this kind of capability has been on Mac for decades but for the great majority of people that have just adopted Mac as their new platform it's pretty cool stuff.