Speed, Boot time, game performance, System tools
Not on my PC. The only thing it adds is the metro layout.
I only upgrade windows if it improves on performance without horrible trade off's like a scrambled OS for the sake of being different.
Or when i need it in order to put my new hardware to full use. But dx11 is still the norm in PC land and console land now, which makes windows 8 a useless OS to have in my opinion, as it adds nothing towards the mix on this department in comparison towards windows 7.
Unless windows 9 adds something drastically visually through dx12 like dx11 did in comparison towards dx9 i will upgrade. But even then, SP1 and second generation dx12 video cards will have to be released. As most of the time the OS and Cards are garbage in it's first revision.
If it adds nothing, windows 9 have to improve my performance in comparison towards windows 7 and not mess around with interfaces. Or i will be happily ignoring entire windows 9 and wait on what windows 10 will bring me.