A rock and a hard place
Apple are caught between adding new features and making the old stuff work well.
IMHO I think that Apple have been too busy try to add new features and updating the GUI and have not given enough time to clean up things after them.
My favourite version of OS X was Snow Leopard; it was relatively small and fast. What it did; it did well.
I only moved from Snow Leopard when new apps that I wanted were only supported on more recent version of OS X; and then I moved from Snow Leopard to Mavericks just before Yosemite shipped because I wanted to last stable version of OS X.
I think that Apple needs a Snow Leopard moment again; i.e. clean out the crud and ensure what is there works.