> Developers will move over their apps. Any major bugs will be found and shook out.
Wasn't this why the iOS6 beta versions were released months before the final release? Now that iOS6 is officially released, Apple is encouraging all users to update and there are quite many average Joes and Janes who don't read the tech press and these warnings. These users are the ones that face the most confusion if something is missing or changed (YouTube, Maps).
Since I installed the update as soon it was available, I don't have first-hand experience of when users are prompted to update. Based on previous experience and some update guides[1], the prompt will appear automatically when connecting to iTunes.
[1] http://ipad.about.com/od/iPad_Guide/ss/How-To-Upgrade-To-iOS...