Remember the Xbox 360's Red Ring of Death saga? Microsoft does, and has taken steps to avoid a similar overheating debacle with the Xbox One. Under ordinary conditions, the Xbox One's fans will not run anywhere close to maximum speed. Thus, if something unusual happens, such as a blockage of the machine's heating vents, the fans are able to rev up to a higher speed, possibly removing the heat issue without any intervention by the user.
If increasing the fan speed fails to solve the problem, the Xbox One can also switch to running in a lower power state that requires almost no air flow. This is likely to cause trouble for high-demand applications like games, and Microsoft isn't yet sure how it will communicate this state to the user. Finally, if neither of these solutions work, the Xbox interface will alert the user of the problem, interrupting the device's current activity but allowing the user to take steps to protect the hardware.
All of this sounds far more useful and informative than a device that powers down and presents its owner with a mysterious red ring. Of course, it's still up to users to provide a healthy working environment for their gaming devices. Keeping space around the Xbox One's air vents and taking out a duster from time to time will hopefully prevent any of these anti-overheating measures from being needed in the first place.