A friend of mine that designs buildings said that the weight of the HVAC/communications gear on the top floors plus the impact and fire basically made the weight above the plane hits more extreme. When the superstructure failed. And when the top floors came down it was like a hammer hit the top of the building. He also compared it to watching a black belt in karate chop several boards. Not sure what he meant by that.
My son works as a architect a few blocks from the trade center. He says that engineering codes require that the steel structure must withstand an hour of fire -- which they did.
Many people escaped. The stairs were not blocked. I saw video of people descending the stairs. there were working lights in the stairwells this time.
It will be a while before we know the death toll, but it appears that those who began evacuation immediately had a good chance -- except for those above the fire.