I can't tune my power saving parameters. I can't have ten workspaces that I switch between easily. I can't use the custom symbol inserter I wrote, which is way faster than any other I've used on _any_ platform.
I can't get my work done as effectively. I also wouldn't enjoy it! Using windows annoys me constantly with little things: the way the start menu works, the way applications lay themselves out on the screen, the way the login screen works, the network preferences and device listing being buried.