Bashiok, Community Manager Blizzard Entertainment, wrote a detailed post on how the Diablo III team approached the enormous amount of feedback after announcing that followers would accompany your hero only in Normal difficulty. He went into further details about how followers will function as well.
"When we first announced the follower feature we revealed that these unique characters would accompany your hero on an adventure to save Sanctuary, but that they would only be able to survive in Normal difficulty." Bashiok said. "The amount of positive support the follower feature received, as well as the number of players who wanted to be able to keep these companions throughout the game — even into the toughest difficulties — left us wondering if it would be possible."
He indicated that the game will become "brutally difficult" in the later stages, so players should really take advantage of their followers.
"Followers won’t follow you into co-op games because you’ll already have the superior firepower of your friends to help you, but playing alone you’re going to want to take advantage of their benefits. We’ve made the later difficulties of the game brutally difficult, and we realize that for those attempting to tackle these later difficulties alone, they’re really going to actually want some additional support in the form of the followers."
Bashiok also brought up mercenaries, saying that they will differ from Diablo II based on previous criticism.
"Some players didn’t like their experience with mercenaries in Diablo II. We took feedback regarding mercenaries very seriously when designing Diablo III followers, and they differ from mercenaries in a few key ways that we think set them apart and resolve many issues."
This time around, there will be no resurrection or cost after a follower's death, making their upkeep "far less intrusive". Similar to how Oblivion handled the mechanic, when a follower's health lowers to a certain point, they simply take a knee for a few moments and then jump back into the fight.
The other big change players can expect to see is that when their hero dies, so does their follower. "These aren’t characters that can hope to compete or continue on without you. While some players prefer to be the lone wolf taking on the forces of evil, our intent isn’t to dilute the hero aspects by adding more wolves to your wolf pack." Bashiod reasoned. "We want followers to be an extension of your bad-assery, not a liability. The followers could almost be considered automated buffs/damage skills, but of course with quite a bit more flavor and customization options."
Bashiok encouraged people to comment on what they think of followers during end-game and what their purpose should be.