From Diablo Wiki
IGN: D2 had a relatively large story that was kind of broken up. Are you still planning on doing the same kind of chapterized settlements, or are you building one epic tale that starts and doesn't really stop until the end credits?
Jay Wilson: Our system is still in progress. It's pretty close to being locked down, we've just got a couple of little details we're working out. We do have acts, like Diablo 2 did. They serve a different role now, they're mostly to break up major chapters, major elements of the story. But they're not necessarily a box for content, because we break down even more so than we used to. We take an act and we break it down into smaller chapters, each chapter has a goal and everything. But the overall act structure is still in the game. Between each one there's a cinematic, it's a way of saying, here's where the cinematics go, here's where we go to a new environment you've never seen before, here's where we branch to the next step in the story.
One of the things we didn't like about Diablo 2 was that…the story was basically the same story in each act. You showed up in the town and the townspeople said, the demon went thataway. In here, every act is different. The first act is a little bit more of a mystery, there's this cult that has been operating in and around the rebuilt Tristram, you're trying to find out who they are. Act two's a little bit more of an intrigue and deception-focused story, because Belial is in there. Act three, there's a lot more.