The most obvious part of Mac OS X that I already love is the protected memory. As I mentioned earlier, I've needed it many times already, and it has worked flawlessly. Once Apple works out all the glitches in the OS, Mac users should truly have a rock-solid, stable operating system that never ever crashes. The Dock is growing on me. Except for that annoying aspect where apps can slide underneath it, I am starting to really like the Dock.
Classic applications run very well and are just as fast as Carbon apps.
In fact, most applications running under the Classic mode seem more stable and solid than the Carbon apps I've been trying. Really, if you haven't tried it yet you should. Even with my iMac playing MP3 files it still understood my voice commands when I held down the ESC key. I've since disabled Voice Recognition in an attempt to reduce the system's slowness, but once Apple figures out how to truly optimize OS X I'll undoubtedly re-enable it. I owned the Public Beta, and have to say that Mac OS X 10.0 feels an awful lot like it.