Account Security option added - just a few old settings like Changing Password, removing your messenger password, changing LIVE IDs for your account, requiring password at sign-in and a Xbox LIVE pass code, just a move around of the UI.
Manage Exercise Info option added - this allows you to (optionally) tell Xbox the following: your height, weight, year of birth, and gender. Information for it says: "These attributes are used by Xbox and some Kinect games and apps to improve estimates of your exercise stats. Exercise stats include fitness scores from calorie burn, time played, physical intensity of movement, and feedback from physical activity."
This can be removed at ANY time if wanted.
Avatar Store option added to Dashboard in Social section, replaces Social Apps option
Not much needed to explain behind this, the Avatar Store option takes you to the Avatar Store while the Social Apps option was removed as it was just the App section with downloads for the Facebook & Video Kinect App.