I didn't think the original question was all that silly. Sure, the obvious answer is that the towers fell down because they were hit by huge airplanes full of jet fuel. But you can be sure that current and future architects will be considering these questions to a greater degree than they ever did before; not becuase they expect to build buildings that are able to withstand anything thrown at them, but because they want to make sure to build them as strongly as is reasonably possible, and they have a new set of considerations (and, unfortunately, a new test-case) to factor into their thinking.

One thing that I have not seen mentioned, and that WAS talked about before 9/11, was the folly of having the US military headquarters in a building that is probably more recognizable from the air than any other building on earth. It is an enormous five-sided structure, built in the 1940s when we DID know a lot about what aerial bombardment can do to buildings. Eyewitnesses on 9/11 reported that the plane that hit the Pentagon first soared over downtown Washington, and there was speculation that the Pentagon was a secondary target chosen because the pilot could not identify the White House from the air. I don't know whether or not this is actually what happened, but it seems possible. It's worth considering that there are other ways to make buildings less vulnerable to air attack, besides encasing the structural steel in concrete.