Mostly in usability and UX. So good that both Microsoft and Canonical have went on the record, stating they're trying to emulate it.
In addition, some others. Grand Central Dispatch. Time Machine. Spotlight.
Spotlight was on Linux (Beagle). Time Machine's just incremental versioned backup (borrows some licensed technology from Sun); such already existed, even on Macs. Very nice interface though, and excellent usability (beats the heck out of random scripts). Grand Central Dispatch is "yet another implementation" of an existing idea.
The UI (which I like), many old Macheads would argue, is a step backward (especially the Finder) and is not as consistent as the old Mac systems though much prettier (IMO).
Again, stability, yes (especially compared to < OS 7, but even when compared to OS 9). Polish, absolutely. But the only real innovation (outside of business ones) I can think of is the decision to build on Job's NeXt and NeXtStep.
[Edit: Leaving the forum for a while, and, with some luck, for good, so I won't be able to respond, whether the above is shown to be incorrect or correct or a bit of both. My final - I hope - comment here would be that personal computers have come a long way, but they're still way too difficult for the average Joe and Jill, even desktop OSX. Perhaps smartphones and ebook readers and the like, or something else entirely, will provide an easier way for the basic "computer things" that most want or need to do. Maybe desktop computers will improve. Either way, here's hoping for better things to come. It's been a blast (of sorts). Have fun. Be safe. Take care of those who matter most. Share a smile and a laugh with a stranger. Hold the door open. Read a book. Drink enough water. Sing. And then smile again. Cheers!]