Originally Posted by biknut
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?
The headset is the bearing and cup assembly. That is the main difference between threaded and threadless.
Threaded headset bearings are tensioned by the threaded cups on the fork.
Threadless are tensioned by the center (aka preload) bolt.
The other difference is that stems on threadless forks are mounted to the outside of the steering tube, while threaded are mounted inside.
They both have their advantages and disadvantages. IMHO, I believe threadless is superior. With threaded, your whole steering assembly depends on that dang wedge bolt!