I'm way more interested in the Mac than iOS. Kind of sad it's more of after-thought than iOS now. I knew when it was presented first it meant iOS would take precedence. And it's kind of reflected in that we're not seeing as many people download 10.9 and post pictures, yet at least. Although, I guess a download and install takes more time than for iOS, but still I think iOS is where people's interest is at.
As a long-term Mac user, I'm most curious to see if some nasty bugs introduced in Lion and carried over to Mountain Lion are still there. I'm less interested in big features and more in just a solid, good operating system that works. I can live with linen and leather texture I don't like. The bugs are what annoy me. Things that would excite me would be seeing if there are any changes to TextEdit, QuickTime, etc.
Or little things that annoy me, like does the horizontal scroll bar in the Finder still cover the bottom item when you try to click on it (the bottom-most item is partially covered by the scroll bar but becomes almost totally covered when the scroll bar magnifies)? That happens to me all the time.