Originally Posted by dutret
Everything except some peoples aesthetics.
Actually, threaded has some advantages such as much easier height adjustment.
Threadless has been mainly driven by manufacturers, as they only need to make one fork length. You can find many threaded set-ups, which are stiff and easy to maintain etc.
IMHO its really a personal choice, but the whole bike industry has pretty much moved to threadless. The choice in threaded is getting less and less, and nearly all the good stuff is NOS.