Subjectively it feels like Apple bugs have become larger and more prevalent, over the last few years. That and IMO clean OSX/iOS installs don't quite feel as polished as they used to. (I stopped using Apple products, except for a MBP, for a few years and recently started using them again, and the MBP still runs 10.10 for precisely this reason) The last solid OSX release was Snow Leopard
They've added features over the years without removing or polishing them; there's Launchpad which was added in a period where OSX seemed to lean towards becoming touch-friendly, but it didn't replace any existing feature (iirc) and just feels off. Might just be me though. Notification center? Don't use it.
