A cool hidden new feature of OS X Mavericks is that the new OS allows Macs with ambient light sensors to detect user movements and delay sleep mode. The feature was first reported by a Mac developer on Twitter, who incorrectly assumed that at first the iSight camera was tracking a user's movement. However, other Mac devs quickly found, and The Verge confirmed, that it is the ambient light sensor located next to the iSight camera.
The ambient light sensor doesn't actually track movement itself, instead registering the changes in light resulting from that movement to detect if a user is in front of the Mac. When it registers changes in light, suggesting user movement, it resets the idle time clock in OS X accordingly. It's a pretty cool feature and another example how its usually the little things that are among some of the coolest features of an OS X upgrade.