Originally Posted by scotto-
Ummm, the main difference is the headset.......
the stem is the other difference. One is not neccesarily stronger than the other, but the non-threaded tend to stay put, once tightened down. The threadless are just as adjustable, if not more so than threaded. If you want to heighten the stem (raise the handlebars) you just leave the steer tube longer and add spacers. You can also flip-flop most threadless stems for even more adjustability.
I'm not sure I understand what you mean by headset. Headset, is the bearings and races that fit between the frame and steerer tube.
Are you referring to the frame head, as the headset?