Moving beyond the source list, I would also add that there are other UI areas where Apple seems to have stagnated. Sheets are a good example. They're clearly a great idea, but yet they fall down completely when you layer them. How many apps use a sheet as a file save dialog, only to pop up a modal dialog on top of that sheet when a file save needs additional input or fails for some reason? Drawers, too, have been completely neglected since 10.0, and they definitely need some serious work or simply a replacement.

These are the unsexy areas where Apple, as the platform vendor, should be putting in the work (both in design and user testing) so that the application developer doesn't have to.