I think the reasons WW works are:
1)They follow a modified food exchange/diabetic diet. In the olden days it was closer to the exchange than it is now, and I think the Core program is basically an exchange diet. But even the points are founded on that. For people who need structure, this is a way to give them portion control. Since the exchanges are based (somewhat) around macro-nutrients and macro-nutrients have specific calorie counts, it is an easier way to control calories. And provided you don't go too deficient in calories or skimp on protein, you can steadily lose weight provided you stick to it or there aren't underlying health issues.
2)The support system and advice. Giving people support, accountability, and strategies for dealing with dieting issues.
3)Because their weigh-ins are considered data for things like the National Weight Control Registry. Not all weight loss programs have this advantage. In fact, at one time WW was the *only* weight loss system that was provided outside of clinical settings that had this advantage. I don't know if they've opened it up a bit, but this clearly may make WW appear to have benefits that other diets (WOE, whatever) don't. I am not saying that WW is not successful at helping people not only meet their goals, but maintain loss, but only considering WW weigh-ins as credible data (and when it comes down to it, it's a business with every reason to fudge the numbers as any other business), you make them appear to be more successful than their competitors, even if their success rate isn't any higher (or is even lower) in reality.
One of the first diets, and definitely the first formal diet, I was on was WW. I remember we had a sort of deal a meal thing. I was too young to be on it formally. I was just following it along with my mom. She lost a tonne of weight, and I lost two pounds over the course of two weeks. She was totally convinced I was cheating and took me off it. I didn't cheat at all, but I gladly gave up. I was hungry all the friggen time, but it gave me one of my favourite snacks: cheesy toast. One of the breakfasts was 1 slice of diet bread with a slice of cheese melted on it. That was the only joy during that miserable two weeks. I remember eating dry canned tuna. Gack.
My sister loves WW and is fairly successful on it. Just like any other diet, if she goes back to eating the way she ate to make her fat, she gets fat. The secret is making sure people don't go back to their old ways, no matter the diet.
ETA: I don't know if I've ever posted these here but:
http://www.candyboots.com/wwcards.htmlThe one I was on was from the late 1980s, but the 1970s ones always make good viewing.