A new feature that was introduced with Ice Cream Sandwhich was the Face Unlock Feature. Although it was a cool feature to show off to your friends it was never really that usefull. It wasnt really all that secure either due to the fact that it could be tricked with a smiple photograph of the person.
But when you upgrade to Jelly Bean you notice that the Face Unlock feature has a new security option. Liveness Check which requires the user to not only have the right facial features and proportions but now they must also blink. This was added to stop the Face Unlock from being tricked by a photo. The feature is still a little rough around the edges and makes unlocking your phone take longer and is a bit inconvenient. Hopefully the Liveness Check feature will be smoothed out in a later android update.