There are many mysterious things about September 11, but any would-be conspiracy theorist should be aware that certain things are highly unmysterious (and that regarding them as evidence for a conspiracy makes you look unhinged).
* US airline security was appalling before 9/11. Their are documented cases of people who managed to accidentally carry *guns* in their hand luggage, never mind small blades.
* The success rate of a air defence systems at spotting threats is not as high as people would like you to believe (this is why Star Wars is a stupid idea, among other things). If the air defence system’s controllers hadn’t been informed of a potential threat from hijacked airliners used as weapons, the threat might not even register at all.
* Flying a 767 or a 757 isn’t an easy job, but it’s not an enormously hard one – and the difficult bits are things like landing and flaps, not airside steering. Purely on the basis of my MS flight simulator experience, I managed to do some reasonably impressive-looking things in British Airways’ real 767 simulator a few years ago. The reason pilots train for a long time is because if you mess it up then everyone dies, so a 95% chance of success isn’t really acceptable. The terrorists appear not to be too worried about people dying.
* The WTC towers were specced to withstand a plane impact, but not to withstand a full planeload of kerosene setting them on fire. While passers-by didn’t think the towers would fall, experienced structural engineers did.
* The towers were also specced so that if they did ever fall due to external force, it would be in the form of a perfectly downward controlled-like demolition. This is the case for all new skyscrapers – everyone concerned really wants to avoid playing Manhattan dominoes.
* Liking strippers and drugs doesn’t stop you from being a crazy religious fundamentalist, as a cursory glance at various American TV evangelists might reveal.