Somewhat intangible, but it's a feeling of how much time has been spent on the software. In the case of Mavericks, there are quite a few display glitches in the built-in apps (flickering, delayed updating of item content, etc). A package that was seeing attention from developers, these would be fixed, as such things are usually relatively simple to resolve.
It's a long time since I worked for Apple, but back then, their development processes were always quite open to developers taking on bugs and running with them, especially cosmetic and "fit and finish" issues. In Mavericks, I don't see signs that the developers were given that extra time, or alternatively, that they cared as much to get the product "right". Compared to the level of rework and finessing that you see in iOS builds, Mavericks looks "unloved": enough is being done to close the bugs, but no more.
I get the feeling that these days, to be assigned to "desktop" in Apple is not something developers want.
As a reference point, I also use Windows 8.1, and since the end of the Windows7 release cycle the Microsoft OS has moved ahead of OSX on "feeling solid"; truth be told, it's only Adobe Illustrator, Terminal and the *nix command suite that's keeping me on OSX these days. (the lack of Illustrator on Linux rules that out)