Operating systems will just be taken for granted, just like electricity, the telephone, the railways and cars. They will be self-healing to a greater or lesser extent, with simple user interaction and interfaces. They may even be 'baked-in' to the hardware of the day, or 'embedded everything', if you will.
Remember as coders, you create other peoples' realities and experiences, what you decide an OS to be, then it will be so. I don't think the 'rise of the machines' will be anything to worry about. Just look at what they were predicting in the fifties, home cleaning robots, you know the sort of thing I mean.
I disagree with Charles on the 'intelligence' issue. Yes an OS could be classed as an intelligent implementation by the coding team, but it will not be intelligent per se. It will give users a perception of intelligence. It will not be able to make a meaningful relationship between ice cream and strawberries for example, because it will never have the 'experience' of ice cream and strawberries.
Yes, many-core and parallel computing may bring more 'grunt' to the show, but it will never become the compelling experience that is human interaction. But, bit by bit (excuse the pun!), day by day it will be improved.