I think it's the fact that the strapless dress is so fitted that keeps it from being a trainwreck.
For example, check out this thing I found on a quick GIS.
It's formless enough as it is even with the lady trying to pose for the picture but it's passable. However, add in drooping shoulders and you may as well start wearing garbage bags.
Also, I hate babydolls and ill-advised empire waists badly enough as it is, but by going strapless you can take one even further out into eyesore territory.
Again, just imagine how extra bad this will look once the model stops posing her heart out.
The next abomination is so terrible that I'm not even going to try using it as an example for anything, I just stumbled upon it while searching for the previous entries and was dumbfounded by the possibilities this thing has when slouching.