-Threadless is stiffer.
-1-1/8th is lighter because they can make the steerer out of thinner tubes while still retaining superior strength.
-Swapping stem/bars if you ever decide to go to the track is pretty darn easy (not that it's not with threaded, it's just not any harder).
-Carbon steerer tubes are only available with threadless forks.
-While it's easy to find threaded stems/forks for cheap now, who knows how easy they will be to find in 4 years.
-Threadless can be annoying if you swap saddles and you need to lower/raise your bars.
-You can't get a stem that'll fit OS bars in threaded.

As for looks, I like threaded, but I'm slowly leaning towards threadless.