Here's my take on the whole thing. For months, the top two questions people kept asking here have been (1) Where's George K, and (2) Why haven't they started construction on Avatar? It just happens to be that - out of the blue - George K. steps out from hiding to talk about Avatar. Suddenly, it's all very clear now, for me. George K. has taken ownership of the Avatar project. This is his baby now - the first project he will supervise from start to finish under his watch as the new WDW president... And he has to get this one right because this is what everyone will look to as a measure of his performance. He needs to make a good impression on everyone who's watching - fans, Bob Iger, future possible boses such as Kathleen Kennedy or Tom Staggs, stockholders, all the WDW cast members working under him, the Orlando Sentinel and other local media, etc.
So, mystery solved. George K. took ownership of Avatar and he's been mostly at the model shop and sitting in on WDI meetings, giving his say on every little project detail, telling the Imagineers to add that, change this, delete that, and pushing the Imagineers to create at the best of their abilities. He probably walked into the project from day 1 of his new job unsatisfied with the work thus far and they basically had to start over. There might have been multiple project restarts.
That explains the delay. It also explains what George was doing all this time in hiding!