My theory is that the reason WW works so well is that its basically a simplified version of calorie counting. Instead of "this is x calories, y grams of fat, and z grams of fiber", it tells you "this is x points". Plus it allows you to eat certain foods that are considered 0 points.
Seriously, when my dad was on it, he LIVED on the Zero Point Veggie Soup. It was actually 1 point a serving, since he added some 1 point peas, but it was filling.