I, too, found it difficult to choose: Unix inside, multitasking (incredible) and protected memory (almost no application crashes and [for me] never one which has caused a restart) are truly astounding.

Fast User switching is nice to allow many users quick and real access to the same machine, and the column view was nice for easy navigation (double clicking in OS 7-9 to drill down into folders drove me to FinderPop (thanks Turly) for which I paid double the shareware fee). The native PDF is also sweet, but the others I don't notice that much [if I weren't a user of CTVirtualDesktop, I'd need Expose].

Warning!!! Will Robinson! Subjective Opinion coming!!! Every sentence should be preceded by "This author thinks that..."
The Multitasking and the stability brought by protected memory were necessary to keep Macintosh on the face of the earth. However, when it comes right down to it, if it weren't for the Unix inside, OS X would have just been a new Macintosh OS to deliver to the core Macintosh users (like myself). The fact the Macintosh (or XServe) is escaping its old boundaries and appearing in places like the Virginia Tech cluster, in the Human-Genome project, and the US Army, and the fact that the Macintosh is being taken seriously again in the media is because of Unix at the core.

Also, without the Unix core, this site and its forum would not likely exist (and I would not have 'met' a number of very kind and generous people as I have at the forums). Therefore, I had to vote for "Unix in the core."