Just to weigh in with my two cents...
I also feel that the flight controls are effective anti-terrorists weapons, anyway they can be used to get the hijackers off their feet.
Here's where I think cameras in the cabin can help the pilots, as you must be able to see what the hijackers are doing to know how effective your flight maneuvers are. I think cabin depressurization is an excellent defense as well. I think this can be done without putting the passengers at too much risk. Again, the cabin cameras would be very helpful here.
I also don't like the idea of bullets flying around in the cockpit (or cabin). A tranquilizing dart gun (works good at a distance), or tazer, or other weapon that can subdue a hijacker quickly, without putting a hole in the fuselage or damaging some other system, would be great.
I don't really like the idea of killing a hijacker (unless HIS actions reduce his chances of surviving to zero), so I'd like to see some method of restraining even the most determined hijacker on the aircraft, so he can do no damage.
I also like the idea of putting a strong door on the cockpit as I feel that saving the pilots, is like being a parent and putting the oxygen mask on yourself first before you put one on a child. The aircraft HAS to remain under the control of the pilots at all costs, so all can survive.
BTW after the events of Sept 11th, if a pilot calls for the male passengers to rush a hijacker, I'm there in a second.