In one sentence:
Unless you are very careful you wind up with a photo of "lots of nothing."
For example: half of the photo is a pretty uniformly bland sky or similarly uninteresting fore-ground. A whole lot of who cares with your subject way off in the distance.
After that crop to what you would have gotten with a 20-24ish lens. So why not have moar megapixels, less weight, and moar-good-optics and just shoot the 20 or 24?
The really wide FOV makes it a bit harder than normal to keep crap from casually creeping into the corners/edges of the image but this usually manageable.
The DOF is always "whoa a lot" so you can not rely on the background going out of focus to force attention back into your subject.
These characteristics/problems/challenges are still there with the 20 or even 24, they are just not as difficult to deal with.
When you shoot the 14mm lens "right" it is amazing. It does really well in confined spaces (Cubicles or other very small rooms, closets, cars, etc) or when you are outside just remember to fill the frame with your subject* which usually requires getting VERY close. In crowded cities this is actually a good thing - by the time you are far enough from the bull on wallstreet to get it all in with a 28mm a tourist grabs its balls... The weird perspective shifting the lens does is also a cool thing in the city. Making people look larger than buildings and whatnot.
If you have not read it, read the Ken Rockwell article on ultrawide angle lenses. Take it with a pinch of salt, 'cause he is Ken Rockwell, but it is one of very few articles on how to shoot ultrawides.
*yes, photography 101, I just find I need to make a very conscious effort of it with this lens.