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Jay Wilson has penned a new developer diary entry over at the Diablo III site in which he discusses some of the crucial systems that the team is looking quite hard at and which will undergo some changes. Wilson's team has been criticized for spending too much time altering systems rather than just releasing the game 'as is'.
Wilson's contention, however is summed up nicely here:
Our job isn't just to put out a game, it's to release the next Diablo game. No one will remember if the game is late, only if it's great. We trust in our ability to put out a great game, but we're not quite there yet. In addition to finishing and polishing the content of the game we're continuing to iterate on some of the core game systems. So all that said, I'd like to provide everyone an update on some of the systems we're currently working on.
Some of the systems being reworked include: