I agree that Snow Leopard was the most recent version that focused on seriously improving the under-the-hood stuff (but whatever did happen to Grand Central Station, which sounded genuinely interesting?). A lot of the additions made since seem primarily intended to steer the user to one or another of Apple's other moneymaking enterprises, usually iCloud. I won't say these aren't useful to many individual users, but it's high time that the development team returns to basics and starts thinking harder about how to help us squeeze more and better performance out of the gear we have.
And I'm amazed that Apple let v.10.1 get out the door without fixing the iPhone/Mac Bluetooth pairing issue. At the moment, Yosemite's highly touted integration of the iPhone with the Mac via FaceTime just flat doesn't work.