Diablo 3 Runestone System:
Seems they've also added an updated follower system this round.
The Followers will all appear at a specific point in the game one at a time and you have to earn their trust through loyalty quests; think Mass Effect 2. Wilson mentions that if you do the Followers personal quest but you don't want him to follow you, you can send him to town where he'll always be available for you to play with. There is no penalty for choosing one Follower versus another; it's all about how you like to play. Plus, if you decide that you want to fly solo you can opt to not have a Follower at all...
...But how exactly will the Followers work in combat? Player won't be able to actively control them like they would a pet; instead they have a complex AI that drives them. How players contribute to Followers is the abilities, weapons, and items that the players give them. Also, unlike hirelings in D2 where it was a pain to heal them, any health or mana pots that players consume will also heal their Follower if they're nearby for the full benefit (yes!!).