This is a Blizzard game, there will be at LEAST a year of fine tuning before they get everything down. Starcraft 1 took like 3 years of major balance changes to get that right, and that is basically a sport, with real money on the line in tournaments. WoW will require balance changes as long as they are adding content. As everything they add subtly changes the balance.
For Diablo III they will also require a lot of balance changes like every Blizzard game, There are 7 million+ people trying to exploit the game and and there is no way to anticipate everything that they will try and every combination that is possible. It will take lots of time and fine tuning before it is right. Just be grateful that Blizzard take the time to actually do it, a lot of devs will just release a game and maybe fix the really game breaking stuff. Blizzard take the time to try and get it right and will spend an incredible amount of time watching how the game evolves and fine tuning which is why their games are so popular.
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