In a 1990 study published in the American Journal of Clinical Nutrition, women controlling their eating had slowed their metabolisms down to where they were eating 620 fewer calories a day than their more slender but unrestrained eating friends. I think this is a big one that you don't hear too much about. The parents of a friend of mine decided she was getting fat and forced her into dieting, a poor diet at that. She wasn't fat at all. Once dieting began, she actually began to gain weight and was always tired. This was the worst thing she could have done, in my opinion. It's a bit difficult to lose weight when your body isn't burning anything and you're tired all the time.
It's called the setpoint, and your body will adjust over time to the amount of calories you eat. If you cut back too far the body will actually slow your metabolism down to compensate. Exercising regularly can actually adjust the setpoint up some.