Bethesta's team of 100 or so people made The elder Scrolls V: Skyrim in 3 years. Less than half of the time d3 has been in production. They developed an entire new revolutionary engine and and a massive game all in this time. Skyrim's content and environment detail is downright amazing. I can almost guarantee it's content and amount of quests will dwarf that of diablo's. And its a 3D environment, unlike diablo. Sure d3 has 3D character abilities and other things, at its core it's still a 2D game. If i had to guess how long it took Bethesta to make that game, I would have guessed 6 years or more. But it didn't.
Another thing people say is that Diablo is an online game. Do you think it really takes an extra 3-4 years to produce online hardware? No.
This leaves no question about it. Diablo's team must be extremely small or they're just extremely lazy which most likely isn't true. A game like this should in no way take 6-7+ years to make. It's just ridiculous.