These are useful qualities. And my OSX's have been up for months at a time (aside from having to reboot 6 times to install Canon printer and scanner drivers - why?). Two are web-servers (internet and intranet) as well as doing a useful day job (you can do that with 8 cores).
But I can't think of any must-have functionality provided by OSX over OS9, maybe why it takes some of us years to move from OS9 and before.
Microsoft have it the same, Win 98 and Win 2000 still make up an sizeable volume of their corporate user base. And you can see why, 98 was fast and quite reliable, 2000 did it's job well. XP meant new hardware for nearly all, just at a time when there wasn't budget to upgrade thousands of PC's. Somehow nearly everyone managed to get off NT.
It's not the OS, but Google Earth is must-have functionality for me that I can't get while running OS 9. Of course now I can't get it in Panther either, and that will be what drives me to upgrade to Tiger.
I waited three years after OS X was introduced to use it. I just couldn't bear the idea that after 20 years of using Macs, I'd have no clue how to solve problems in OS X. Nor could I imagine what a non-issue that would turn out to be when I finally dipped my toe in (at Jaguar, soon upgraded to Panther).
Bah, Google Earth, of course. Maybe one of the reasons I bought an iPod Touch, just so I wouldn't have to switch from Panther to Tiger on my MDD. I had thought that the images available in Google Earth were available under Panther with Google Maps (had the Google Maps car outside my house yesterday for 20 minutes taking street pictures, hopefully all my playing about outside with my 19-month son will bring fame, even if they fuzz my face). Likewise, my 20+ years of Unix is largely unrequired looking after Mac OSX, although it does help with programming projects.