Cider
I came up with an idea for an OS a while back: Real simple, no jargon, no error messages, an OS for Kiosks, Morons, Drunks, non-geeks, technophobes, etc. I called it Cider (Apple for drunks). HUGE buttons on a desktop with no other features, buttons like

o Stuff, Look Up Stuff, Games, Go Shopping, etc.
I was selling to the public and the Imac drive of 2000 was into the spring and sucking the dead pondscum off the bottom of the market. Most persistent user errors?:
Not Quitting applications (I'm serious, people would crash their machines or load them to the point where they'd function at half speed or worse)
Not trashing or failing to empty the trash for months on end.
re-installing the system with the "save and install" option and getting lost. Resulting in a schitzoid, nearly unrecoverable mess.
The thing that makes OS's such a pain in the ass is the average IQ of a consumer when you take culture and Dialect shock JUST USING THE MACHINE into account. Otherwise intelligent folk would do stupid things because they're neither Geek or Academic. The "Killer OS" will have to be self-repairing/restarting and rediculously simple. OS X is close but still too full of jargon for the poor average (Make it go) schmuck who buys (say) a cube.
Case in Point: one (otherwise intelligent ) couple bought a cube and didn't take off the scratch-film packing material because IT WASN"T SPECIFICALLY IN THE UNPACKING INSTRUCTIONS!!!!!!!
the poor little cube was stuck in a restart loop because the film of plastic was setting off the power button on both the machine and 15" LCD. I removed the film and discovered that they had fried it's start-board into that loop. Our store had to eat 1200.00+ for their idiocy. Any technophile would know better, but you have to keep in mind, not everybody is and, The average idiot is now using one of the most convoluted, evil OS's ever devised .
Mac OS for the PC would really give Bill a run of it. Market analists be damned.