Official support from Apple for the multiple desktop support they include in the OS.

Support for standard UNIX tape drives so that standard UNIX backup software works.

Better integration with foreign file systems and standard UNIX apps. It's great that you can get the resource fork of a file with the "/rsrc" suffix, but you still need to write a lot of code to get at the rest of the file metadata... how about generalising that hook, so you could use "/finf" to get the finder-info, or "/bin" to get the whole thing as an AppleSingle file?

An input manager. Programs register themselves all over the place, with contextual menu extensions, services, keystroke sequences, window corners, and so on. How about giving us a preference pane to manage all this instead of having to deal with all these conflicting programs? I want to be able to put frequently used services in a context menu, and shuffle off some rarely used contextual plugins to the main menu somewhere. I want to be able to start the screensaver with a key combo instead of having to hit that corner of the screen in a hurry. There's just too much stuff to remember, Apple, it's bad enough keeping track of what each app does with option/command/shift click sequences. Remember why you went with a GUI in the first place? Recognition versus recall?

While I think of it, how about letting me clone commonly used menu items into the contextual menu as well? That way I don't have to keep going to the upper left of my 17" display or memorise a command sequence...

Oh, and if I turn on "full keyboard" navigation, I'd like that in Safari as well. :)