It's missing basic interface features that have been bog standard in Ubuntu/Windows for years, like any sort of proper window management. Hell, the maximize button actually doesn't work- it has such inconsistent use that you have almost no hope of actually getting what you want out of it. To get even decent window management, you have to buy software off of the app store.
Then, you have the fact taht you still functionally have a folder-based OS, which feels a greater need to present me with buttons to alter the view of a given folder, how icons are arranged, etc- all useful things no doubt, but things you don't change on a per-folder basis that often- while literally not having an "up one level" button. At all. You either have to know the keyboard shortcut, or you have to go to the Go menu. The back button isn't always dependable for this interaction(especially if the finder was just opened), so of course this just screws the whole pooch.
Seriously, the most I can get anyone to offer as to what makes OS X such a great operating system, is that it's Unix with no driver problems. If that's the most we can claim, can we stop acting like the interface design is halfway decent? Windows Vista had better interface design than this. That's just sad.