In 1967, the MIT AI Lab needed a multiuser OS for their PDP-6. Existing timesharing systems were slow, expensive, and not well suited for AI and robotics work. So the MIT hackers wrote their own, tailor-made OS.
ITS had no passwords and no file protection... only usernames. It had memory protection, but I suspect that was just a pleasant side effect of using the paging hardware for efficient memory allocation.
What did ITS look like? From what I've seen, it was like Unix (perhaps ITS was an influence on Unix?), but simpler and more efficient. The ITS graphics system was said to be very flexible and efficient... apparently it used the paging unit to provide direct access to the video framebuffer.