You know what I'd dearly love? The ability of MacOS and Windows to support multiple simultaneous graphical users.
You know, like we had back in 1984 with the release of the XWindow system.
It's trivial to put multiple video cards into a modern system, and likewise trivial to hook up multiple keyboard/mouse setups.
Windows has the RDesktop/VNC hackish crap, but it doesn't really work for anything other than very basic graphical display, and still requries a full hardware system to run the RDC or VNC client on.
It makes no sense to require a full-fledged PC on every desktop, and even the "thin client" thingies are pretty much a full PC anymore.
It's ridiculous that the Mac/PC world doesn't have this at all.