Remember, even the sims respond better than the actual a/c and in the case of the second a/c to hit the WTC I believe you can actually see that the landing gear was down which would also effect the mobility and steerability(sp?) of the plane. Anyone who has ever flown what we in the aviation community refer to as "pigs" heavy a/c, understand the slow response time to pilot input to the flight controls. Kinda like trying to stop an a/c carrier on a dime, just ain't gonna happen. Also, the lack of a proprioceptive (sp?)sense (seat of the pants flying) points to someone who only trained on a sim and never felt that feeling in the seat of his pants that comes from severe banks or dives.
Hitting the building is the easy part, assuming you are lined up.
The line up. Hitting a building is like a bombing run. You need to make a turn to the final run-in course and not overshoot.
Speed. The second plane was clean (flaps and gear up), which means it was travelling over 200 knots (probably more like 300). Flying at that speed and hitting the target requires more skill.
The planes took off out of Boston, and were travelling to California. Depending on when they were taken over, they had to be flown for some time, descended from a high altitude, etc.
Interestingly, the planes were modern (757, 767). These new planes have sophisticated navigation systems, with multiple inertial systems updated by GPS. Very accurate. Most older planes like the 727 don't have this. With GPS, and good coordinates of the target, it makes it easy.
A skilled private pilot, using a good PC flight simulator, could prepare for this, to include learing how to operate the navigation system, but PC simulators don't provide the side vision needed to estimate when to make a turn to the run-in course. If an initial point was programmed into the naviation system, the terrorist could have used it to lead the turn to the run-in course.
In short, I doubt some slack-jawwed yokel from the streets of middle east did this. This is not driving a truck into a building.
According to an eyewitness report I heard on the news, the plane that crashed into the Pentagon actually hit the ground first and skidded into the building. Since I can't think why a kamikaze would do such a thing, my only guess is that the pilot didn't know how to visually judge ground clearance on the large aircraft.