My favorite features so far:
Time Machine -- Backing up has never been so easy and so sexy. This thing has already saved my life as I accidently deleted some priceless files for the577.com.
Spaces -- At the office I'm constantly working on as many as four projects at a time. Finally I'll have a space (virtual desktop) dedicated for each project instead of having a clusterfuck of windows.
Screen-Sharing -- Got a problem with your Mac? Just hit me up on iChat and we'll start up a screen-sharing session so I can show you how to fix it.
Hasta la Vista!
More Info: OSX Leopard