A Lot of Missing Core Columns
Remember, a good section of the core remained after the outer walls and floors went down-- the "spire". Where did all those core columns go? There should be at least a hundred of them, and they should be on top of the pile. These columns are also big enough to stick out clearly from the rest of the debris.
Interestingly, the west wall of the north tower-- the side closest to the camera-- went down without major debris falling on top of it. In other words, this wall (or at least what is left of it) was BLOWN away from the tower and not simply pushed over by debris.
The last point is that these outer walls were 200 feet by 1350 feet ON EACH SIDE. There is a LOT of outer wall steel that seems to have disappeared.