In a recent interview with IGN, Diablo III Lead Game Designer, Jay Wilson talked about the length and pacing of the game:
"Honestly it's similar size to Diablo II. There are some differences here and there, exterior environments are a little more diverse, dungeons are about the same. Even the way the Acts increase in length and then scale down. We intentionally did that again because we thought, some of that was done to ship Diablo II, but we thought it had a good feel to it to reduce the length of later acts so that you feel like you're accelerating towards the finale."
Jay Wilson also commented on the progress of the game, the quest system, skills and, the latest feature to be revealed, artisans and crafting. You can read the full interview here: page 1, page 2.