What does the Atkins diet and Skyfall have in common? or to be more broad, what does any action or spy movie have in common with diets? A Macguffin.

These movies have very similar structures: you have a good guy, a bad guy and the reason why they have come face to face. This reason, the thing that differentiates one movie from the next, is called the Macguffin. How can we sit through countless bond films and still be entertained, is the slight difference between each movie. 
The same can be said for diets. They usually promote certain foods (the “good foods”) while vilifying others (you guessed it, the “bad foods”). The Macguffin is the “catch” on how this diet is different from the others. All of these fad diets are low calorie- it’s the way that they get you to restrict your calories that is different (or the “macguffin”). They may not state outright that the diet is low calorie, they are more likely promote some “miracle” food or supplement that is the “trick” to weight loss.
The reason why diets are not a healthy or smart way to lose weight, is the fact that they are not sustainable. They are designed to work only in the short run and can only really be followed for a few weeks or months. 
Also, they can recommend eliminating healthy foods, such as milk and alternatives, fruit and whole grains. I suppose you could say they all have the same unhappy ending: weighing more than what you weighed when you started dieting. 

Here are some popular diets and their Macguffins:

Atkins diet: good guys = protein and fat; bad guys = carbs (fruit, grain products, other starchy foods).  
Paleo diet: good guys = protein and fat; bad guys = carbs (grains, dairy, other starchy foods).
South beach diet: good guys =low glycemic index foods, good fats; bad guys = high glycemic index foods and bad fats like saturated and trans fats. Certainly less restrictive than other diets but low calorie never the less. 
Montignac: good guys =low glycemic index foods; bad guys = high glycemic index foods. Oh and lots of complicated food combing to limit what you can eat.
Eat right for your blood type: good guys = “depends on your blood type”; bad guys = “depends on your blood type”. Basically, each blood type will be limiting certain foods to get calorie intake down.  

Many of these diets have common bad guys and good guys, only proving that creative labeling foods as good and evil is not the best strategy to manage your weight. 
It’s funny how, despite having all of these diets, that there still is an “obesity epidemic” in most countries. Perhaps it is because they do not work? or because they are not “the answer”? 

Quebec has an interesting site to visit regarding diets and weight loss plans. Here is an interesting pamphlet if you are considering taking on a diet. 
Equilibre is another great site for more info. 
If you are a health professional, joining the Canadian Obesity Network is free and their website is great!