"Diablo 2" introduced the concept of followers. They were basically AI mercenaries who would follow you blindly into battle, dealing out damage every once in a while, but generally just hanging out and getting paid. You could customize your follower with a weapon or an item, but that was about it. In "Diablo 3," followers are getting a lot more attention, as evidenced in a new trailer from Blizzard.
Unlike the faceless mercenaries of "Diablo 2," the followers in "Diablo 3" have names and back stories. There are three individuals you'll encounter, a Templar, a Scoundrel and an Enchantress, and each has entirely different abilities and benefits to help your player character. The Templar, for example, is ideal for ranged classes who need a bit more melee support to keep the hordes at bay. The Enchantress, on the other hand, can create illusions to distract your enemies.
While you'll meet all three in your journey (they play a part in the main storyline), you can only choose one to travel with you at a given time. This means picking the right follower for the job, which can be tricky.
Since the followers are meant to compliment your main class, you can further personalize them with items, weapons and skills. It's somewhat similar to the pet system in "Torchlight," where your pet could be taught basic spells. In "Diablo 3," though, it's a lot more complex, as each follower has 12 unique spells, and you'll have to narrow that down to 4 chosen spells.
Blizzard is saying that followers aren't meant to be a big part of game's strict balancing system, as they become somewhat unnecessary in multiplayer games. In singleplayer, however, followers allow you to fill in the blanks where your primary class might be a bit lacking.
Player customization has always been a major part of "Diablo," and it's great to see the follower system getting the same sort of attention. Will the followers be making an appearance in this summer's external beta for "Diablo 3"? Here's hoping.