Diablo II was consistently popular for over a decade thanks to its immense replayability. At its core, D2 was a game about building new characters and gearing them up by any means necessary. Every enemy in the game was a loot pinata just waiting to be popped, and players farmed endlessly for a few sought-after unique items. You almost never found an item that was ideal for your particular class and build, but you could usually trade for what you needed via trade channels and forums.
Blizzard claimed that the auction house was intended just to streamline this process, but when Diablo III launched, it was clear that the entire game had been designed to make the auction house almost necessary for progress. The fault here lies not just with the concept of an auction house but with the game designers.
That's right: I'm here to argue not only that Jay Wilson was right about the auction house ruining Diablo III but also that it was his own damn fault.