Macworld lists out the ten most significant contributions of the OS to the Mac experience, but things like the Time Machine and Spotlight are on there, and of course, those weren't introduced until much later. The OS itself was actually announced six days after the year 2000 started, and the Apple Expo announcement was the beta reveal. Steve Jobs had just come back to the company, bringing his NeXT experience with him, and OS X made waves early on for being a Unix-based operating system. The Aqua user interface was the other big feature of the OS right away -- as Jobs said at that first Macworld Expo, "when you saw it you wanted to lick it." Customers found that out immediately: the beta product itself was released in stores for US$29.95.
I'm sure we'll have more coverage of the past ten years of OS X as we move forward into next March, but it's pretty amazing to think how far both we and Apple have come over the last ten years already, all the way from OS X to iOS and everywhere in-between.