I'd have to say it's the built-in LittleLite plugs on the 9000 Pro. The lights themselves are optional but I couldn't live without them anymore. They illuminate the entire keyboard so it's easy to see all of the buttons/labels for quick access to all the other "Kool" features, and they provide enough illumination that I don't have to have a seperate light on my music. Never saw a keyboard with this feature before or since.