(Although you're right that scalable fonts would be REALLY nice to finally get in an OS, 100% working and backward compatible with existing apps.)
Off-hand, I can think of PLENTY of things that OS X could offer in the future to vastly improve the operating system!
A few quick examples off the top of my head?
1. Improve the look/feel and feature-set of the Address Book and Mail apps! Address Book looks like a bare-bones skeleton of an app ... not anything Mac-worthy at all! Plus, not all of the fields you can put in there sync cleanly to equivalent fields supported by other manufacturers' cellphones or devices. It'd be nice to at least indicate which of these fields were not necessarily "transportable" by color coding them differently or something.
2. Improve networking with other operating systems. Even in the very latest revision of Snow Leopard, my friend recently encountered bugs mounting NFS shares from Linux machines using the feature Apple moved into "Disk Utility". (It used to work fine in Leopard, the old way they handled it.)
3. Give up the ridiculous idea that windows should only resize from the bottom right-hand corners! Nobody I know has ever given a sensible argument for how that can possibly improve the user experience. When I need a window resized, I want to be able to grab it from ANY corner that's most convenient and have it respond the way I'd intuitively expect. Why artificially restrict it?
4. MANY file management tasks could be made easier if OS X got creative, offering new tools for them! For example, did you ever want to rename a whole list of files in a directory so they all started with a certain prefix, or maybe move/copy all files in a folder starting with certain letters (or ending with a certain extension)? Right now, I see no good way to do it in the Finder. I guess you could write a whole Applescript or Automator script to accomplish it, but that seems like way more work than it's worth - for something you probably won't need to do again the exact same way in the future. Heck, in good old MS-DOS on the PC, decades ago, you could do much of this with a simple command line! Like "MOVE *.TXT C:\MYTEXT" to move all files with a .TXT extension to that folder.