I still hate messy hair. But what is messy hair?
First of all, there are many beautifully done pictures out tyhere, where the model's hair is messed up. The big difference is that, the messed up hair was intentional. It is part of the look. Here lies the difference.
Messed up hair vs. messy hair. Messed up hair is part of a look. Messy hair is plain carelessness, lack of attention to details, and bad art direction.
What exactly is messy hair? We can only let the audience judge on a case by case basis. Every picture will be different. However, if the hairs are going in 9 different directions, chances are, it's messy hair.
The photographer, if taken the time to stand back and look, will always knows, the hair was part of a messed up look or is plan messy hair.