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In an action Rpg you clear dungeons, waves and waves of enemies for the chance for loot to drop to strengthen your character.
Usually you would need to sink a bit of time into the game to farm for gear upgrades to clear the content.
IN passed games it was always possible to play solo or in a group, work through the game at your own pace and finally finish the content.
In Diablo 3 it is impossible to clear content without buying other players drops that were found one to three acts ahead of you.
In Diablo 2, you could progress completely on your own, it was a grind but the gear you recieved was good enough to clear content.
Lets say you have a character in Inferno difficulty act 1. This character can farm all day, everyday and never, ever get the correct gear to do act 2... This is a broken system.
This turns the game into a gold farming game and forces players to use a trading system/AH (something that should be optional btw) just to progress in content.
I know many people have no problem with this. They shut up, farm the millions of gold and buy gear that they shouldnt have seen until Act 3 or 4.
THe problem is this takes away from one of the best aspects of a A-Rpg, the loot finding!!!! If you're only playing to see your gold value go up you're missing out on the what makes this genre great.
TLDR Version: You can't find the gear to progress in the game so Diablo 3 is broken.