Now that Blizzard feels their engine is solid, it’s time to move on to creating content for the full game. Jay Wilson, Diablo 3‘s lead designer, says the game engine is in an advance stage of completion and much of the art team has moved onto Act 2. Don’t read into that, he said game engine.
Blizzard is obviously not hurting for money, given the success of World of Warcraft and they’ve always taken their time with projects to make them solid products. Blizzard’s name is on the line and they’re brand consists of three major franchises: Warcraft, Starcraft and Diablo. When you produce so few games you’re expected to do them perfectly.
How do they do it? As Wilson says, “If we haven’t rebuilt something five times it really doesn’t deserve to be shipped. That’s pretty much our policy.” (eurogamer) While a game engine is the foundation for a great game experience, the content is king. Blizzard is deep in the mire of content production so, although the engine might be solid, the final product is probably still far from complete.
End result, expect a high quality product once again from Blizzard.
After four years of development, 
I’ll be preordering the Special Edition as soon as it is announced.