It's got major user interface changes. And lots of end users who don't know that the basic operational features of a computer can change will find out, they can.
And their patterns of use will change.
Brain cells will burn.
Will the users shrug it off or even cheer, as Apple expects/hopes?
Will users respond as they did with Windows 8 by rejecting the update?
Will they have an option of going back to a previous version or is this update irreversible?
Or is it mandatory? Can you not update your devices? If so, how?
I haven't done it yet, but I know two people who have. One says OMG and the other says the changes are cool.
But tomorrow we find out for real if Apple can rock the boat.