Load several web pages simultaneously while you are re-arranging your bookmarks and downloading files in OmniWeb and this will be obvious. Unix also supports multiple processors. If you use X and both processors get used automatically. The current implementation only supports two but we expect more in future. Unix provides a true multi-user environment.
Users log in to their own custom environment.
They can set privileges to allow access to other users on the local disk. Each user can have their own programs, preferences and documents. This is implemented naturally within X, unlike the modification to 9 which doesn't provide a particularly efficient model for this functionality. The difference is in 9 they are tacked on to a system which was never designed to use them, so reliability and efficiency suffer. The Unix basis of Mac OS X ensures the functionality above is built in to the core of the system and it works far better because of this.