This sound like a generally beneficial idea, but I have two concerns.
The first is about system availability. At least in current versions of windows updates require non-trivial amounts of time and bandwidth. I now use my computer to stream live video regularly. I want to be able to tell windows that it is not allowed to take up bandwidth or try to reboot my computer in the middle of game 7 of the world series. In a similar vane, I can see some real outrage from users if they are in the middle of a video game and windows decides to steal focus to ask you if it can reboot. (Oh wouldn't it be great if windows could catch up with Linux where almost all the upgrades can be done without taking the system down)
The second is about breaking things. Several have mentioned applications breaking after updates; a concern, but one that at least in theory can be minimized by good testing. My concern is breaking users. Like many here I have older relatives who are not comfortable with the whole computer concept. Every time facebook changes the UI I get desperate e-mails asking where things went or what this new doohickey means. Changes in UI that may be small and obvious to digital natives are major impediments to less skilled users. My great fear is that I will constantly be fielding phone calls from Grandma asking where the button for her Mahjong game went this week.
To quote an awful movie, "with great power comes great responsibility." I will be one of the late adopters of Windows 10 as I wait to see if Microsoft can handle the responsibility it seeks to take on with this new direction.