Popular Mechanics pooled together a team of experts to debunk 9/11 myths.
A few of those I've already discussed, not here. Last semester I took a course called Engineering Ethics. One of the lectures, we watched a video on an ethics case. The man who designed and built the WTC was presenting himself as a case study, on whether he did enough to prevent the WTC collapse. It was a very moving video, and very interesting (structural wise). These are facts that I got from the video:
When the planes hit, The fuselage literally fell apart, and 'blew' into the building, blowing the fireproofing material right off of the structural steel frame.
The company that administered the fireproofing material did a half-assed job, in some cases, only half the fireproofing material was found to have been installed.
One tower collapsed on the inside, and it "implodes". The other tower collapsed on the outside, and that one you see the "top" of the tower staying there for a few seconds, before finally starting its own path down.
The Jet fuel burned only for a few seconds, but ignited the raging office fire.
The raging office fire caused some of the floor support beams to 'sag', and, as they were designed only to resist sideways motions, not up and down motions, fell from the connecting bolts, and when enough of them fell, the floor collapsed.
Computer models at the time couldn't accurately model building fires and what it does to the structural integrity of the building. The building was also designed to withstand a hit (think: Empire State building airplane hit) from the largest airplane at the time, which was considerably smaller than the plane that ACTUALLY hit.
Despite that, the plane withstood the impact VERY well, just not the ensuing office fire (computer models couldn't model the jet fuel, only the impact of a big heavy object).
The resulting discussion as to whose fault it was, not for the initial impact and fire, of course, but why the buildings themselves collapsed was very interesting. There were a few innovative designs in the WTC. They had to ease buildings codes to make the design feasible. If they had attempted to build the WTC in the same way that the Empire State Building was built (heavy structure and the like), there wouldn't have been enough room for rental spaces, and the building owners would have lost money, so why bother building it?
I'm sure wacko-loons will still claim it's all Karl Rove's fault.