Originally Posted by DannoXYZ

QUILL STEMS
- heavier
- more flexible
- easier to twist in fork
- easier to adjust for height
- one-piece wrapping clamp tends to be quieter, although tougher to change bars
- looks cooler when horizontal without too much of a handlebar-drop

THREADLESS STEMS
- lighter
- stiffer
- harder to twist on fork (due to larger clamping area)
- tough to adjust for height without future planning during installatoin
- might not look as cool when adjusted for comfortable handlebar-drop
- easier to change bars, but 4-bolt clamp can squeak
- pretty much the only way to install a stem on an aluminium or CF steerer tube

There are pros and cons to both, you just have to pick a list of criteria that's important to you and find the system that best fits those criteria.