Quicklook - Just push space and see just about anything without waiting for the program to load. Yes, I know Apple didn't come up with the idea but making it a built-in part of the operating system was a great move.
3D Dock - I know many people disagree with me, but I think a 3D Dock looks really cool and adds really nice eye candy. And since eye candy is one of the things Apple is best at...
Spaces - This adds great functionality to work systems. Personally I don't use it because I try to have no more than three windows open at once. I prefer Expose, but that's just me.
Huge Icons - Eye candy; It's all about the eye candy. Just like coverflow.
Trash Can - I really love the new sound effect for the trash can. If you have surround sound or headphones you can hear the trash bounce from one side of the can to the other before it hits the bottom. I know this seems really weak but I like the attention to detail.