threaded systems adjust on a skewed cartesian grid, where x is the stem length and y is the range of motion offered by the quill length and the steer tube. this makes it pretty easy to play around with your riding position, even well after your bike is 'complete'. threadless gives you the same adjustability, IF you leave yourself enough excess steer tube when you built the thing to move the stem up and down in the spacer stack until you are happy. otherwise, your option is to keep buying/swapping stems of similar but different angles and lengths to try and find the coordinates you otherwise would have reached by adjustment along the steerer axis. in reality, threadless gives you more options, because most threadless stems can be flipped for a different angle, and spacers are available in basically any thickness if you look hard enough.

couple that with a stiffer interface, lighter componentry, wider availability of cartridge-bearing threadless headsets, wider availability of open-face threadless stems, wider compatibility with bar sizes like BMX and 'oversize', and you have a strong case for a mechanically superior system.

having said all that, i still don't think it looks as nice (with exceptions of the cinelli alter, 3t mutant, and thomson elite), and i still have a bicycle with a loose-bearing threaded headset and a nitto stem.