So the new Friends app does a couple of new things, which includes the ability to not only have friends (as usual), but also followers. Think of a follower as someone who is a fan, that can check out your performance in a variety of games and stay connected with you, while you don't have to follow them back (think Twitter). There are two separate privacy levels between a friend and a follower, so it's up to you what they can see as a follower.
“A great driver in ‘Forza Motorsport 5,’ for example, could have a huge group of thousands or even millions of followers all tracking their performance in-game,” Rob Lehew, Xbox Live Program Manager, said. “Those followers can have this driver on their personal leaderboard so that they can constantly keep track of how they stack-up, and they can potentially connect online with the driver to get into a race with him.”
There are also new types of Achievements to be had, which in addition to the traditional type, now include time-based challenges. These challenges will be available for a certain period of time, so Achievements horders, will want to make sure they accomplish them during the given time frame in order to earn that Achievement.
"As a nod to our stellar community, many challenges require a community to work together towards a common goal. Imagine, for example, a game releases a weekend objective that requires players to cumulatively secure an objective “X” number of times in a three day period. And every person who participates and helps build towards the challenge’s goals unlocks that Achievement and earns its reward," noted Nelson on his blog.
Now take a look at Major Nelson's video above, which includes more info on the new Xbox Live features.